I think that everyone coming to Lisp for a period of time experiences this discomfort. It's a completely new way to think. I don't believe, however, that only certain people are capable (with practice) of comfortably programming in S-expressions. I think it just comes down to who is willing to experience the discomfort for the length of time it will take until they have their first "aha!" moment.
For me, I had some minor "aha!" moments while programming in Lisp, but not enough to switch. It was only later, when I wanted more powerful compile-time capabilities in C and when I joined python-ideas and started seeing the plethora of syntax proposals (often followed by lengthy debates which usually led to no changes) before I realized just what makes Lisp a more powerful language.
Good observation about Lisp vs. Smalltalk, by the way. (I also wonder why Smalltalk isn't more popular...)
http://shootout.alioth.debian.org/debian/benchmark.php?test=...
My current pet theory is that only a minority of people feel comfortable thinking in s-expressions. For the majority, trees with many levels of nesting aren't a natural medium for expressing their thoughts. (I don't know why this would be, but it's worth pointing out that human language has a very low tolerance for nesting (in fact you almost never see it go beyond level 1)).
I know two very good programmers who gave Lisp a good try and just don't feel comfortable in it. So I don't buy the inferiority argument either. Interestingly, both of those guys strongly prefer Smalltalk, which is arguably as close to Lisp-with-all-those-trees-unwound as anyone has got.
Edit: I'll add that if the nesting theory is correct, I feel lucky to be in the minority that finds it comfortable to build programs that way... what I'm able to do in Lisp far outstrips what I was able to do before.