I don't think there's much wrong in the PDZ graphics as such, but there has been variable quality shots lately.
The likely explanation is that they are simply from different levels/worlds. Some like the jungle look very good, while some of the inside levels look bland.
Overall the detai in levels isn't that high, much less than say in GoW.
Also maybe it's the lack of over the top shiny effects that makes the game look a bit duller in comparison, and that there hasn't shown any big cool monsters, just a few human enemies.
The latest videos do look so much better than what was shown at E3, now it actually looks like a next gen title. Also in the videos the grapics did show some neat shader effects that weren't over the top as in some other more showy titles.
One thing that worried me when looking at the videos was that the gameplay at least in those short clips seemed very uninspiring, and the levels a bit repetitive. Though it's unfair to judge by such short clips and not really knowing if there are other gameplay elements than just straight standard first person shooting
