Oh I dont know.. lets see.
Quake 1 - first true 3d FPS.
Quake 2 - first true usage of 3d graphics cards
Beyond that each itteration introduced new ways to not only produce graphics and audio but was unbeatable in terms of pure graphics technology.
I agree Quake 4 doesn't really introduce much new, but it's a great game in and of itself IMHO. Then again Raven has never been known for introducing games that bring with them fantastic new technology, only using the worlds greatest gaming tech, in the form of an Id Software engine, and making great games with it.
If you seriously don't think Q1-3 brought anything to the table that was new, you probably werent around to play those games when they first came out; because they blew the rest of the industry away.
EDIT: Oh and yea Q4 gets much better beyond the first 30 minutes. In the first 30 minutes you think your playing a pretty strait forward FPS, nothing exceptional. After that you start encountering better creatures, going through better designed areas and start getting upgrades to your weapons and such. It definately improves and by the end, is a fantastic FPS. Maybe not as good as FEAR, but for a strait FPS, it's a great one.
This comment was edited on Nov 24, 10:42.