Ok, here's one: it's slow. ObjC has dynamic typechecking and methods invocation is achieved via something akin AmigaOS' BOOPSI dispatchers, with the exception that ObjC dispatching can be a lot slower due to the fact that the "methodid" is the hash value of the method name, and computing hash values is slow.
This depends heavily on implementation, dynamic method calls in C++ can be just as slow. AFAIK Apple's implementation of Objective-C is very fast, basically the system keeps a cache of dynamic method calls to avoid to recalculate each and every time hash values, it works pretty well.
Add to that the strange syntax, completely foreign to the C syntax, it uses for classes interfaces/implementations and methods invocation, and you get another complaint. Whooo, we've got already two of them!
Yeah, that can be a problem, Obj-C syntax was borrowed from smalltalk so you can like it or hate it. Personally, having used BOOPSI a lot (which is quite influenced by smalltalk IMHO) I find it nice. Apart from class/methods definition (which are different from plain C even in C++) the only addition to the language is the method call which is much different from a C function call and thus makes it pretty clear where you are using dynamic calls and where you're not.
