Just compared my two. It's really not so simple, and the old screen is not better in *any* way.
Subjectively the 2g screen DOES look brighter. I had them both set at 50%, auto brightness off. I also compared at 100%. The 2g always "looks" brighter.
But looking beyond that initial "that one seems brighter", and really comparing.. they are actually about the same brightness I reckon. The difference is that the 4s has MUCH higher contrast. That means the bright areas are similar, but the dark areas are much darker - meaning the maximum brightness is around the same but the 2g puts out more light. That's a very bad thing - you want blacks to be black, not medium grey. Because of that, the 4s screen looks MUCH clearer than the 2g, even ignoring the higher resolution. I'd say you get the same brightness, but more contrast, which is a huge improvement.
Besides that, yeah there's the huge resolution increase. The 2g looks blurry, text is all smeary (and very hard to read when it's small). The 4s is super crisp.
The 2g has tons of 'light leak' too. When viewed from an angle, blacks turn a bright blue. This doesn't happen on the 4s. It's a sign of a very bad screen.
Also, look at the colours. The 4s looks pretty normal. The 2g? Safari's grey toolbar is lilac! The metal strip on the iPhone in the screenshot is pink! How can that possibly be better?!