typeclass - What's wrong with GHC Haskell's current constraint system?

Question

I've heard that there are some problems with Haskell's "broken" constraint system, as of GHC 7.6 and below. What's "wrong" with it? Is there a comparable existing system that overcomes those flaws?

For example, both edwardk and tekmo have run into trouble (e.g. this comment from tekmo).

question from:https://stackoverflow.com/questions/12805335/whats-wrong-with-ghc-haskells-current-constraint-system