I liked Assassins Creed, had a blast playing it, and I actually thought it was over too quickly. It could've done with another couple of cities. I'm hoping the sequel builds on it in interesting ways. I'd definitely class that game as underrated.
Oblivion, when you first start playing, gets an 8 or 9 out of 10. Only after having spent hours in it when you realise its glaring flaws, does that score start to drop, I'd say as far as 5 or 4/10. I think this is why a lot of reviews gushed over it at the time - a lot of reviewers won't have reached that point yet. However, with the right selection of mods installed you can easily bring the score back up again and surpass the original game, and if you aren't prepared to do this then you should sell your PC and get a console. It's one of the things I like the most about PC gaming, is the mods.
Haven't played Halo 3 so can't comment on that, but I really enjoyed 1 and what I played of 2, so if it's just more of the same and Bungie didn't fix what wasn't broken I dare say it would please me.
I think when some people see something get really overrated, for some reason they take it too seriously and it angers them, so they purposely under-rate it to "rebalance the scales" or something. That is what you're seeing.
Also, I think everybody with more than 20 brain cells recognises that Yahtzee is first and foremost a comedian, and he deliberately focuses on and picks apart games negative points for comic effect, and as such should be taken with a pinch of salt. If I'm using reviews to judge whether I buy a game or not, I go to Metacritic and choose a couple from the high end, a couple from the mid-range, and a couple from the lower range of scores there, and that gives me as balanced an outlook as it's possible to get without actually playing it myself.