First thing I should note when reading people's comments: Yahtzee is pretentious. Yes, he's funny as well, and that's the biggest reason I can watch these. However, he is also pretentious. He makes Adventure Games, where the biggest market is a bunch of other pretentious people. Being someone that looks to design games but with a different background, I can recognize many of his points and agree.
On the topic of Halo 3, Bungie dropped the ball on story altogether. I think this was a horrible conclusion, and after hating Fall of Reach I've said "bollocks" to the rest of the books. In fact, some of what is considered canon actually ruins the story in the games for me. Is the story to all three games horrible? No, and anyone that says such a thing can shove a roll of socks in their mouths. The setting is interesting, and the subtle hints of the origin of Halo, the Flood and information about the Covenant is all great from the start as long as you pay more attention than to "guy in green armor goes to save world", which is all people ever examine when discussing the story. Halo 3, however, has nothing subtle in there aside from the terminals. They even leave some questions unanswered. Once more, ball has been dropped seriously badly on this one with story.
As for gameplay, Yahtzee is and isn't correct. I don't care about the multiplayer much either, to be honest, and I also hate the dicks of Xbox Live. Yet I enjoy the single player quite a bit. Granted the improved combat of Gears of War has diluted my enjoyment, but Halo is unique. The only A.I. that sucks is that which takes over when the Marines drive. Somehow that part got even worse than the previous games. However, the rest of the A.I. works very well and often surpasses that of the competition. While the Grunts provide a comic relief, you also begin to take them seriously when you try to charge a dozen of them and you explode from the Needler. What's wrong with giving your enemy a little personality for a change instead of having them just be big bad growling foes? Plus, while all of the elements of Halo 3 may have already been done, they've been put together in a well designed package. There is no better dual-wielding system, vehicle hijacking works well and is rare enough because the enemies aren't dumb enough to intentionally give you opportunity, tearing turrets off can be a blast and there are few games where grenade pick-ups explode in massive chain reactions when left on the floor. Average is, honestly, an understatement.
If you want nothing but games that make you think, then naturally you won't get this. However, Halo 3 is a combat game, and the Halo series has done combat very well. Of course, Gears dethroned it and Call of Duty 4 likely will as well, but so what? It went from being the best to not. Who cares? Part of the reason it's no longer the best is because it pushed the market anyway. UT2K4 suddenly includes vehicles in there, despite UT2K3 originally not having them? Hmm, I wonder if that has anything to do with Halo's sudden use of vehicles? Etc. etc. and so on and so forth.
Halo may not be the most amazing game ever released. Hell, Halo 3's engine is the strongest of the series, but the story plays it out merely as just another of the series instead of the grand finale it should've been. However, given that I can't find saved films, skulls and, on console, anything like Forge I'd have to say the game has more than others can offer. That doesn't mean it deserves the high scores, but it certainly is above average, even if by a slight.
Again, I like Yahtzee's reviews. Love them to death, in fact. However, I don't take all of his criticisms seriously since he is a pretentious PC gaming fanboy who can't recognize that "dumbing down for the console 'tards" at least introduces more intellectual elements to the one aspect of the gaming market that matters: consoles.
EDITUS: I should note that I do not mean to slant Yahtzee, either. Again, I love ZP, and I've been slowly going through his archives of FullyRamblomatic, and it's even better that ZP brought me to this site at all. I like the articles here. However, from my observations, it is a simple fact that he is pretentious when it comes to gaming, hence why he can nitpick so badly.
Then again, I'm likely guilty of similar things since people on Wii60.com had serious issue with my rating Metroid Prime 3: Corruption an 81/100. Then again, I wouldn't have minded them dumbing Metroid down if the combat was actually worth more of a damn.