Esperanto's problems include an alphabet with unique characters and a gendered grammar that, if you're charitable, is quaintly old-fashioned but, if you're blunt, is sexist. I can't imagine it being accepted as an International auxiliary language (IAL) without addressing those flaws.
Alas, Esperantists may have killed all hope of progress for Esperanto in 1905 with a declaration that stated, among other things, that the basis of the language should remain the Fundamento de Esperanto ("Foundation of Esperanto", a group of early works by Zamenhof), which is to be binding forever: nobody has the right to make changes to it.
Esperantists today seem to be divided between Raumists (who promote Esperanto as a language and culture deserving of respect for its own sake) and Finkavists (who promote Zamenhof's dream of Esperanto as the world's IAL).
Because conservatives blocked reform of Esperanto, IAL liberals moved their allegiance to Ido, a major tweaking of Esperanto that still has advocates. But many Ido supporters moved on to Interlingua, which takes a different approach (see Comparison between Esperanto and Interlingua). From Interlingua's Wikipedia page:
...its vocabulary, grammar and other characteristics are largely derived from natural languages. Interlingua was developed to combine a simple, mostly regular grammar with a vocabulary common to the widest possible range of languages, making it unusually easy to learn, at least for those whose native languages were sources of Interlingua's vocabulary and grammar. Conversely, it is used as a rapid introduction to many natural languages. Interlingua is also unusual for being immediately understandable to hundreds of millions of people who speak a Romance language.Interlingua would be a great IAL for Europeans, Quebecers, and Central and South Americans, but it's not so useful for Asians, Africans, and English-speakers because it's not as simple as Esperanto, Ido, or Mondlango (the best proposal for an IAL today, IMHO).
Strongly recommended reading: History of Esperanto, a short Wikipedia article that has some charming details about Zaminhof, and a little about Hitler, Stalin, and the Cold War U.S.A. being suspicious of IAL supporters.
A link for Emma: Interlingua and the Man From U.N.C.L.E.
ETA: Robb Kvasnak and His Argument For Esperanto