One point I don't think many people have picked up on is the fact that Yahtzee isn't calling the multiplayer, in any way, shape, or form, bad. If you enjoy playing Halo on multiplayer, I think that by now it's probably safe to assume that, if you liked Halo multiplayer in the past, you're going to like it here. That isn't the point. Any additions or tweaks to the Halo multiplayer in this game is, in the larger scheme of things, completely irrelevant.
Think about it for a second; let's just say they decided not to put the single player campaign in at all. Halo Reach was exclusively multiplayer. Would you still buy it, even though it's just multiplayer? Sadly (in my mind, at least), there's still a lot of people that would. But the point is that it's not Halo anymore; it's just another multiplayer game with space marines and aliens. Granted, it's a very good example of multiplayer in a game, and so it should be, seeing as it's been tweaked and refined for this long, but it's still more or less the same experience you've had with all the previous multiplayer offerings in the previous Halo games.
In this sense, the only part of any real relevance to the series is the single-player campaign. The campaign needs to be good; this is the part that needs to be judged. If the series cannot stand on it's single-player alone by this point, after four sequels, then it doesn't deserve to be called "good".