Why are criticisms always met with hyperbole such as this? Halo 3 hasn't sold 10 million copies, at least as far as I know. But that's beside the point. As you say, one can't simply judge a game based on sales. There are hundreds of thousands still playing Halo 3, but there are many more still playing Call of Duty 4, Call of Duty World at War and now Call of Duty Modern Warfare 2. Even now, Halo 3 struggles to outpace any Call of Duty game, despite there being three of them on the market.
Halo 3 has a lot of problems. I don't even think the most diehard fans can argue that. I don't think anyone is calling it pure garbage, just saying that it has problems, and is a disappointment in the shadow of Halo 2, which is the premier MP game with a propper netcode and weapon balancing, and Halo: Combat Evolved, which is still the best single-player FPS I've ever played.
True, tripmines did become annoying with friendlies running over them and the carbine was too powerful. But I think it went from one extreme to the other. Having a tripmine that can only destroy a vehicle if it's extremely damaged isn't much good, especially with the skill it takes to lay one out. And the carbine loses almost every battle with a BR.