(Some of the responses here are hilarious. I feel like I'm reading a Slashdot thread from 1998.)
I think it's interesting to compare Objective-C with Microsoft's COM. Both originated at roughly the same time (late 80s). Both aim to add dynamic, object-oriented features to plain old static C. Both provide a thin runtime library and C-based API to bring those features to C. Both use the same reference-counted memory management. Both allow you to dynamically load objects at runtime, and query about the capabilities of these dynamically-loaded objects in a general way. Both have been integrated into multiple different object-oriented programming languages. Objective-C differs in that it is both a runtime library and a language, where COM is just a runtime library.
COM become hugely popular in the late 90s. While it's natural to use COM to implement a plug-in framework for an app, people started to use COM for everything. This led to the deCOMtamination movement of the early 2000s.
I too did a bunch a COM development in the late 90s, just like everyone else. If I knew then what I know now, I would have ditched COM entirely and used Objective-C and OpenStep. But even if one had a time machine and could travel back to 90s with knowledge of the future, there is no way you could convince anyone else to ditch COM for Objective-C. The syntax is unusual to C, C++, and Java programmers, and there are just too many misconceptions about Objective-C floating around (e.g., see some of the other responses in this thread).
Objective-C has many interesting features that make it ideal for doing the kind of large-scale client-side desktop and mobile app development that it's known for. But I think the truely compelling aspect of Objective-C is the cohesion between the language and UI frameworks that make up Cocoa and Cocoa Touch. The frameworks and the language were developed hand in hand.
Objective-C without the Frameworks isn't compelling to people who aren't already familiar with the language, and those frameworks are (now) only available on OS X and iOS.