Actual combat armor with removable helmets for the whole team. This is going to be a war, after all, not a beauty contest. Don't mind if the armor looks cool, though--pink and white colors do not qualify as combat gear, as much as I like Ash.
Modular weapon customization. I'd like to be able to change color, add and remove parts of weapons in addition to simple increase damage upgrades etc. For example, add stock upgrades for the weapon for better accuracy, add/remove laser pointer, upgradable scopes for the sniper rifles, stability upgrades, etc. All in all, an upgrade system for weapons similar to the one that there already is for Shep's armor in ME2 with parts that you can obtain.
More replay value. I'm one of those people who loves to play a game until it gets boring, and I must say that ME1 offered more than ME2 in that respect. The biggest letdown in ME2 for me was that playing the game several times did not allow you to grab another special weapon, i.e. when you got the Widow, you couldn't get the Revenant anymore.
Non-combat parts in main-plot missions. I thought the main-plot missions in ME1 were great. There were parts where you didn't have to fight but where there was actually some RPG. Think of Noveria and Feros. I'd like to see more of that in ME3.
More actual combat operations rather than one squad blasting its way through hordes of enemies. The mission to Virmire and the attacks on the Citadel and the Collector base were some of the jewels in the series so far. You had the feeling of participating in something bigger than your small squad. There was something going on.
Correct military procedure. I know this was discussed in another thread at length, but I'm just going to add it to my list because every time I see the crewman guarding Legion salute with his left hand makes me wince.
Last but not least, at one point in the game I want to see a dreadnough in full action. The space battle of the Citadel in ME1 was one of the greatest things I've ever seen in a game. The Normandy attacking the Collcetor base in ME2 was high quality and enjoyable, but nothing in comparison to the Citadel battle in terms of dimension. Let space burn!

Other than that I have complete faith that ME3 will be a worthy conclusion to the series.