I vote "idiots who are easy to manipulate." Outright corruption in press is rarer than many people like to think -- as in, "here's your brown envelope, make sure the critiques look good."
However, I think that game reviewers are easier to manipulate into giving good reviews than, for example, automotive reviewers. They tend to be younger, more excitable, and more easily swayed by "exclusive access," bling-bling demos, and so on. More, the pressure to get the review out on release day is huge -- and you're much more likely to get butchered if you're perceived as being "out of line" with other reviewers.
For a game like The Witcher that easily takes 80 hours to play through properly -- *and* one that has two zero-day patches, with apparently some non-trivial issues affecting a significant minority of people left in -- there's no way they can do justice to the game. Either they're playing a buggy beta build, or they're rushing through it like mad; either way, the results aren't good.
OTOH with something like Oblivion, they have months of very careful PR management feeding them with "facts" they will be too rushed to properly investigate when writing the actual review (e.g. how radiant is Radiant?)
So, if you're doing a rushed play-through in order to get the review out, you're worried about not getting caught in a firestorm by having a different opinion than everyone else, what do you do?
(1) You take the industry-standard hype level as your baseline.
(2) You want to make sure that you won't get caught singing the praises of a game that doesn't get the fundamentals right -- i.e., that won't run properly, that has egregious UI bloopers, that performs like rubbish, etc. If you trust the studio/publisher to support the game with patches (i.e., it's a big, well-established, well-resourced one), you might go a bit easier on the bugs than if it's a new entrant (to you). Again, you don't want to be the one who made a zillion people buy it and then not be able to play it.
(3) You adjust the final score up or down a bit based on your general feeling of the game, which, of course, is largely determined by whether you like the genre to start with, whether you like long or short games, whether you weight action or story, and so on.
(4) Even if you have doubts about some much-hyped feature (e.g. Radiant again), you won't have the time to properly investigate it, so unless there's something completely unambiguous that you happen upon and can point at and say "look, it ain't working!" you will let it slide.
So, basically, a wily publisher who knows how the press works and has a decent budget to work with will have no trouble working up a great pre-release atmosphere, and will find very few reviewers cool-headed and dedicated enough to actually cut through it... until much later anyway.
But I don't think it's as simple as "paid reviews."