It's not that the syntax is any worse than other languages. It's just unnecessarily different from the typical C++/Java OO syntax that 95% of all software developers are comfortable with.
It's weird to say that the Objective-C syntax is unnecessarily different from C++/Java when it has been popular more than a decade before both of them. When the Objective-C language was used as the basis for NeXT in 1988 it was fairly mature, while C++ was merely confidential in its use and Java wasn't even an idea.
But C++ quickly dominated, won, and was copied my most other languages since then. Apple should have admitted that Objective C was a loser a long time ago and dropped it.
C++ didn't quickly dominate, it took a lot of time for it to become popular, starting from 1991 which is the date of its real introduction to the world. The first decent C++ compiler from Microsoft only dates back to 1998 and in 1999 Quake III was still written in C for example.
Apple purchased NeXT three years before that (in 1996) to use their assets as the basis for Mac OS X. At this time this choice made a lot of sense since they had a complete environment to build a new platform : a language, a desktop environment (NeXTstep) and a whole development environment (project builder, interface builder, etc.).
It doesn't offer anything new in terms of functionality, so its foreign syntax and structure just serves to confuse and drive away development mind-share.
Its syntax is not more foreign than the C++ one, they just used different paradigms. C++ syntax was based on Simula while the Objective-C syntax was based on Smalltalk. There is no syntax intrinsically better than the other, it's just that you are used to the Simula-one and don't want to invest time on the Smalltalk one.
Also the goals of the two languages at their creation were somewhat different, for Objective-C it was reusability, which is very important when you use it in a complete ecosystem like Mac OS X or iOS. For C++ it was large scale development. As always, it's only a question of the right tool for the right job...