Achievements are a really cool addition to Duolingo. I think they have two problems though.
1. There are no rewards once you reach three stars. The problems with this are, some people feel like getting three stars on achievements aren't worth it. If they feel that the achievements aren't worth it, they might not work as hard as they should with learning a language.
2. I think there should be more achievements. Many have probably completed them a while ago, I'm close to finishing them. I know you guys are busy trying to humor our other requests, but please try to fix these things. Thanks!