Andy_K 

I've raced on:

-2008 Kona Jake 3x10 and 2x10 (~23 pounds)
-2009 Surly Cross Check 3x10 (~25 pounds)
-1977 Gitane Gypsy Sport, singlespeed (~24 pounds)
-2008 Kona Major Jake 2x10 and 1x10 (~20 pounds)

On average, I would say that my results have been indistinguishable between these bikes (uniformly bad). It's much more fun on the Major Jake though. The gears make a big difference on some courses. The weight makes a big difference in how the bike feels and is probably a factor in fatigue toward the end of the race. I don't think weight makes much of a difference in acceleration.

I may be a bad test case, because I'm just slow. I'm certain, however, that the guys at the top of my category could beat me even if they were riding 35-pound Schwinns and I were on a tricked out carbon Ridley. The number one guy in a given race would probably drop a few spots if you put him on a heavier bike, but he'd still be very good.

I do think brakes make a very big difference, in spite of claims that the top guys don't brake much. They do. The trick with turns, for instance, is to carry as much speed as you can as long as you can and then brake quickly to the speed you need for the turn before you start turning. Good brakes make that happen. On some turns you can scrub enough speed just by backing of the gas, but a lot of times you need the brakes.

Probably the biggest difference between a modern CX race bike and an old sport touring bike that you can press into CX service is the geometry. The way a bike handles is all about the geometry. A bike that's built to be comfortable for hours and hours of riding along paved roads isn't going to slither through a chicane nearly as well as a bike that's designed for slithering through chicanes. Again, a good rider can make it work and a bad rider can make it look hard on the best bike, but either one benefits from not having the bike working against him.