What's wrong with this sentence?

There were less people on the train today than there were yesterday.

If you've read the title of this entry, then you've probably already figured it out.  Try this on for size:

There were fewer people on the train today than there were yesterday.

It's an easy rule so let's all try to get it right.  If you can count it, you use "fewer". If it's an uncountable quantity that varies in size, yet is basically one thing, (a collective noun, for those of you keeping score) you use "less".

Less sand.  Fewer grains of sand.

Less water.  Fewer drops of water.

Less waiting.  Fewer line-ups.

Less paper. Fewer pages.

That is all.