Common objections, summarized by the Wikipedia article, "Criticism of Esperanto", http://en.wikipedia.org/wiki/Criticism_of_Esperanto
1.1 Lack of neutrality
1.3 Esperanto has no culture
1.4 Difficulty in achieving fluency
1.5 Esperanto counteracts linguistic diversity
1.6 Special characters
1.7 Not gender neutral
1.8 Unnecessary case and number agreement
1.9 Has not achieved its creator's goal
Esperanto favors Central European learners
1. Justin Rye:
"Many Esperantists have a weird model of political neutrality. English is considered an unacceptably partisan choice as International Auxiliary Language because it's closely associated with (if not original to) the USA, while Esperanto is considered neutral because it isn't a national language (that is, the principal language of a nation-state), it's the language of a harmonious and open community. That sounds sensible enough until you compare it to other world-wide standards such as S.I. units, which became international by becoming national for more than one nation; the neutrality Esperantists value so highly (just like its small-town friendliness) is the mark of a failure. After all, if the EEC had adopted Esperanto as its lingua franca in the seventies, Belgium would by now be full of eurocrats claiming it as their native language; wouldn't that make Esperanto just as politically unacceptable as English for an Asian interlinguist?
Besides, Kurdish isn't a national language either, but that wouldn't make it a politically neutral choice as a global auxlang. Nations aren't the relevant question; what matters is the power-balance between existing speakers and new learners, and that's mostly dependent on how the learners are organised. There are obvious reasons why people might be wary of adopting the tongue of the current coca-colonial superpower, but that isn't the only option – here in the UK we have our own independent standard dialect, and India and Ireland have versions with quite different geopolitical associations. None of these countries maintain Language Academies full of Grammar Police, and even if they did you'd be at liberty to set up a new standard dialect of your own."(http://www.xibalba.demon.co.uk/jbr/ranto/s.html)
2. A.Z. Foreman:
"It didn't take much time looking at it before realized that Esperanto is, in virtually every way that matters, chock-full of features that, thought not all drawn from European languages per se as people claim, nonetheless clearly favor European learners.
Then came the conversations in Esperanto. It became apparent to me that, though many can craft written sentences in Esperanto, a far smaller number were actually able to communicate with any fluency- whereas the number of people who try to learn the language and then give up is quite high.
Still, I thought for some time that it might make a passable Lingua Franca for Europe, even if such a thing was functionally impossible. Several years and one linguistics degree later, I realized that, of European learners, it favors most especially central and east Europeans. (And for some time I didn't realize just how profoundly the latter was the case.) Moreover, the schematic derivational morphology coupled with the propensity of European speakers to import European borrowings has had the unintended effect of giving the learner an even greater burden than most regional languages would or could. At the same time, it contains features that absolutely no language on earth uses and which speakers themselves seem to find weird enough to avoid in many cases, leading to the features being applied inconsistently and contributing to the lexical burden. On the other hand, it's still definitely worth learning for other reasons.
The only person on the internet with a good knowledge of linguistics who gives Esperanto a thorough structural autopsy is Justin B. Rye whose page can be found here. Indeed, it was through long email-debates with him that I came to some of my conclusions. He makes a number of good points (and a couple which seem suspect to me, which I myself endlessly debated with him in a couple long email threads over the past three years or so.) His basic point is that Esperanto is in nearly every way more central European than anything else. I differ in that I think the features aren't central European themselves, but certainly do favor European learners. (But that's academic at best.) Anyway, I will try not to repeat what Rye has said since I don't like typing for no reason. Moreover Rye's observations -which do have a good deal of merit to them- seem to be based not on Esperanto as it is actually used but as it was designed, or else just taking the rules and just running with them. My concern is more with the practices of the Esperanto community. I'll try and make points that he doesn't, mainly having to do with the language's ostensible learnability as actually spoken, its morphological quirks, how in some ways Esperanto actually violates linguistic universals, as well as the outright false, and sometimes downright bigoted, claims made by its more zealous proponents.
... What it shows, both intentionally and unintentionally, is that Esperanto, with all its typological quirks, lexical chaos and strangeness, is like any other language on earth: its origin, features, and behavior are limited and determined by time and space. In its phonology, morphology and syntax, it bears the stamp of its origin as a 19th century conIAL: extreme freedom in what is grammatically acceptable, a clockwork morphological system, inherent sexism and a mal- prefix which doesn't seem to do any good a lot of the time. But as a living language, it has speakers changing it (usually unconsciously) through constant usage. This usage, like any usage, is not schematic or systematic. Morphological anomalies (such as agent-nouns without -isto) crop up, a plethora of synonyms are introduced alongside the regular derivations, syntactic norms become a bit more fixed, and even the evolution of new conjugational patterns (like amantas for estas amanta) becomes possible- these signs of usage are a testament to Esperanto's success as a language people want to be using. Even features that are unheard of in other languages, such as the typological oddity of the mal- words, isn't stopping people from being perfectly at home with common words like bona and malbona, and competitors such as as liva "left" for maldekstra haven't gotten much traction in common speech- precisely because, like any language, core vocabulary doesn't change quickly for any reason: it is mainly the more abstract mal- words (where the problems are not just typological but also functional) which seem to be losing ground in favor of near-synonyms (such as mallibereco). And mal- even finds new uses, such as in youth-slang where trinki "to drink" yields the hip formation maltrinki "to take a piss." As Esperanto continues to live, it is showing itself more and more to be a regular human language, with unique features stemming from its artificial heritage. Despite being not as simple, regular or innately accessible as advertised, it is nonetheless a easier to gain at least a working knowledge of than many other languages (though certainly not all.) To learn it and take part in its culture of 1.5 million speakers, and read its uniquely rewarding literature is an experience worth having. As such, it is worth learning: if only to become part of the community of its speakers, many of whom are among the coolest people I've ever met.
The world has outgrown the circumstances Esperanto (as a universal language) was intended for. In Zamenhof's time, Britain, France, Austria-Hungary, Russia, The US and Japan were all superpowers of relatively equal strength, and so not only English, but also French, German, Latin were all seen as essential components of a lettered European's education (to this one could add the Portuguese sailors had to know.) Today, though, as American English plays an increasingly ubiquitous role (despite the seldom-acknowledged waning geopolitical stature of the United States, American culture has lost little of its prestige) Esperanto, for better or for worse, simply cannot compete. Not that there aren't people who struggle horrifically with English and who are at an unfair disadvantage because of it, but I doubt very much that Esperanto, even if it were in English's place, would really do much better for more than a small handful of practiced language-learners however easy it might be.
Neither Zamenhof nor anyone else of his era could imagine what technical modernity would bring. Indeed, the greatest current prospect for the "family of humanity" which Zamenhof hoped to foster isn't any language or belief system, but the internet- a system allowing contact between any two people with access to a network connection. Even though well over two thirds of all internet users are not native English-speakers, every major programming language is English-based, something like 65-80% of all web content is in English, and one cannot get very far in the IT industry without at the very least a reasonable passive knowledge of written English. Granted, most such English-users, outside of maybe northern Europe, can't claim fluency or even conversational proficiency (even in India, where the official language is English, the number of speakers isn't much above 15%.) But most don't need it. My own experience from traveling in foreign countries whose language I don't speak is that people know just about as much English as they need to. A cab driver can follow directions and talk about prices, a shop-owner can talk about his commodities, a gamer can handle internet jargon, a student of international politics reads the Economist, and a prostitute can likewise follow directions and talk about prices. English today, though it offers innumerable obstacles for learners (including one of the two or three most arbitrary orthographies on earth), is close to what Zamenhof dreamed of in his youth. His mistake was to believe that an international language would be the cause, rather than the effect of cross-cultural information flow. "(http://blogicarian.blogspot.com.au/2012/02/esperanto-international-auxiliary.html)
- there are about 1,000 native speakers of Esperanto worldwide, Wikipedia article on 'native esperanto speakers'
- a thorough linguistic analysis and critique, http://www.xibalba.demon.co.uk/jbr/ranto/index.html