Except it wasn't a "new engine". Wish people would quit saying that. It's a modified version of the old one, with many concessions made in other areas of performance to up the fidelity of other areas. Audio will drop randomly from certain sources, even in multiplayer, where you need it most. Especially in MP, the frame rate is horrible and rarely touches 30fps. The engine will drop to like 1-3fps in some scenarios in MP because its rendering isn't optimized properly (things happening to one player that no one else can even see are still calculated on their client end anyway, for example).
The campaign is carefully designed so that you don't see the mostly nothingness outside of whatever corridor (indoors and outdoors) that you're supposed to be confined to. Previous games allowed for tons of exploration. The skyboxes, which are often only truly paid attention to in campaign, are not on par with previous titles, again to work towards that "great-looking" game you describe. There are excessive amounts of garbage-collection in the game to push the performance as well, which only leads to annoyance and less consistent experiences (better pick up that gun you dropped within 4 seconds or it's gone forever, and so is the ammo, BUT LOOK AT THESE GRAPHICS, MAN!).
Back on topic, despite all it's issues, I personally enjoy playing the game with a small group of friends, and half the time we're just messing around. Competitively, aside from a couple gametypes, the game is not even close to "Halo" and believe it or not, that's a huge reason why so much of the original population isn't here anymore. Can't blame them for leaving when you have to dig deep to find the fun.
