The main problem for me was the aesthetic part: textures, models, animations, sound.
Then, the blocky feel (I wanted to punch the monitor when I got stuck on the pillars of DM6)
The general aesthetics of quake 4 always pissed me off, it's like looking at one of those bad NES titles with garbled backgrounds and shitty grey colour palettes.
I suppose they had to do something different, the LG sound was weird, but kinda of catchy. That blocky feeling went when the slide improved I think. But then that was it's demise, as people couldn't keep up with it. Slide was no sound, so caught oldschoolers out with no sound cues.
But I loved the slide :) if perfected on monsoon, you could slide all the way from Mega jump onto RA with one snakelike slide. That was quite fun.
Wasn't there a shaft changing option?
But back to the q3 better than q4 comment, whatever the graphics (which I thought were nice enough especially single player).
I remember playing 2gd in a 1v1 on monsoon, and he was just hitting rails all over the place, even when I thought I was not visible. (Told me why, which was nice of him.)
So I think that it was pretty fair to good q3 players imo.
If you look at most of the pros, who were at the top of q3, most were pretty good at Q4 and all were very active in it. Why who knows? Maybe because of the potential prize money (harsh to say but potentially the case), or just the plain fun and balance of the game? and that the game was current?