Gravemind80 has produced what may well be the single most exhaustive commentary on the Halo series to date, covering not only comparisons to Halo 1 and Halo 2 including each weapon and every vehicle, but single player and multiplayer level design as well. He caps it off with a series of mostly level-headed suggestions for Halo 3.
Despite his predilection for citing some pretty sketchy sources, he also is familiar with the analyses of Finn, Mothergoat, and Mike Miller.
Certainly not everyone is going to agree on all points; as he confesses himself in the HBO forum post linking to the article, he generally prefers Halo 1 over Halo 2 although he did enjoy the latter, and many of the suggestions amount to "make Halo 3 more like Halo 1". Luckily Frankie has already said that at least in some respects this is going to be true.
There's almost too much meat here to pull out any one morsel as representative, but since Campaign is close to my heart, one particular suggestion tastes pretty good to me:
As for the stage designs themselves, they should be huge. The stages in Halo 2 were supposed to have been really big. In fact, it was claimed that you could fit the entire Halo 1 Campaign into a single canyon in Halo 2 Ã¢â‚¬â€ an obvious exaggeration. However, due to the huge draw distances upwards of 13 or 14 miles made possible by the 360Ã¢â‚¬â„¢s graphics capabilities (see below), thereÃ¢â‚¬â„¢s no reason that the stages cannot span a rather huge distance. Most of the larger stages in the previous games were only a couple of miles long from beginning to end, so I fully expect the stages in Halo 3 to be two or three times as long as that at the very least. Furthermore, I expect the outdoors stages to have parts that are much more wide open than even what was seen in Halo 1. ItÃ¢â‚¬â„¢d be neat to test your sniping skills against enemies that are a kilometer away. I want to see something that gives the effect of Assault on the Control Room or Two Betrayals, but on a much grander scale.
Be warned: this is a very long and detailed read; it might have been easier to digest as a series than as a single long piece, but here it is. Take breaks if you have to, but it's well worth it.