1) Medics and Medivacs. Being able to heal units is awesome. Why was this totally eliminated from multiplayer? Couldn't you have nerfed the Heal ability a lot, or something, without just taking it out? Also, now Terrans have no transport units available in multiplayer (excluding Command Centers for carrying SUVs), and this is definitely unfair.
2) Tech tree bonuses/upgrades from the single player Research Campaign. Yes, I realize it would potentially lead to OP Terran players, but it could be offset by making these upgrades optional / buy-in in game IF you had completed them single player. Right now it's just random which upgrades you can have (bunker with reinforced HP and carrying capacity? check. bunker with a turret? no. raven? check. science vessel? no.) and this would be a way to tie in a person's single player efforts to their multiplayer experience, a continuity that I think would be pretty cool.
An alternate way of doing this is making those tech upgrades linked to alliances. A Terran player would need a Protoss ally to go up the Protoss tech tree.
You could make this more balanced by requiring the research be done in sequence in the order used in the game. That way, you only get the really awesome benefits later-game, when they are less likely to have a dramatic impact on balance.
But I would really like to have mass zerg slow, or auto-vespene, on some maps. :)
3) Co-working construction SUVs. Yes, this opens up the possibility of tower-rushing, but it means that they are spending unit slots and resources on things other than units. I feel like it's a valid tactical option and would like to see it reinstated.
And now a few other suggestions for gameplay improvements in multiplayer:
A) Allow the Zerg Nydus Canals to be shared/used by allies. This would make them up to 4x more awesome/useful. And why shouldn't Dark Templar be able to use those as well as Ultralisks? As we saw in the Single Player, marines (Raynor etc.) can go into the Nydus Network no problem.
B) Allow the Command Center to load other units besides SUVs. It would be cool to be able to load one up with a few marines as well as SUVs to partially defend a base on move (bonus points if they are able to fire from inside the command center). This would also help make Terran bases more defensible against zergling/raider/zealot rushes.
C) Neutral mercenaries similar to the Xel Naga shrines in that they are available to all players (equivalent to the Mercenary Camps in Warcraft 3).
D) Nerf the void ray. Right now there is no way to counter a mass of them.