Originally Posted by
AlmostTrick
Of course there are, and to varying degrees those differences will matter more to some riders than they will to others.
The biggest jump in quality and ride is between cheap department store bikes, and entry level bike shop bikes. (or bikes from some of the better online suppliers, like BD) Above this level, differences are not so profound… especially on a simple fixed gear bike.
Don’t forget too, the power of the Placebo effect. After doling out loads of cash, the urge to “feel” a faster, stiffer, (yet more comfortable!) ride is very strong. And too, when friends or forum members give us the high five of approval for our choice of $$$$ bikes, that only increases the “difference” felt.
Again, I’m not saying there is NO difference, just that it can easily be exaggerated. I'm sure I've even done it.
But our economy is built on consumers spending more, even if that means taking on debt. So it’s all good! 
There might be a small amount of placebo on certain things but I have noticed just test riding bikes the quality difference. I could see a placebo effect seeing difference between Ultegra and Dura Ace or something like that but generally nicer stuff will ride better but the top end and level beneath you probably won't notice a huge difference unless maybe you are counting grams.
A good cog and chainring set up on a single speed will ride a lot better and be more noticeable mainly in the fact it will actually be less noticeable since it is generally quieter and smoother. There might be other things that aren't noticed as much like maybe a set of handlebars or maybe a seatpost. However drivetrain is a bit easier to detect. There are other things like certain contact points and other things that are also fairly easy to detect. Sure again after a certain point you may not be able to tell and it is just about spend-o-nomics.
Department store and some "entry level" stuff is fairly close. If I was held at gunpoint and forced to chose a bike I would go entry level but I would rather at least go a notch or two up from that. The good thing about an actual shop "entry level" is at least it will be put together by someone more knowledgeable using the proper tools. Some shops cut corners but at least can't cut the number of corners that Wally Mart does.