ObjC's main "frying" factor is that it uses a different (and to my mind, superior) set of object-oriented patterns as primary abstractions. For example, subclassing is much less common in ObjC than in Java, preferring to use the less complex delegation pattern as opposed to subclass-and-implement patterns for behavior extensions.

Another problem some people have is that ObjC brings to the table all the pain of C. You've got memory management and bounds checking and unsafe casts all dumped into your lap. It can be pretty painful if you're not experienced in C, C++, or ASM programming.