So far I have very few problems with the current game (or what I can tell from not having played it yet). Yes, there can always be changes for the better, but I won't know that yet until I actually have the chance to play it.
One of the topics that has been on my mind more than any other hasn't been the model of the DT or the shield/bayonet of the Marine upgrade. Rather, I've been wondering if
SC2 is going to have just one Archon (the Twilight Archon) or if we're still going to have the Dark Archon.
The DA was one of the most revolutionary units for Broodwar in my opinion. It had three powerful abilities: Maelstrom, Feedback, and Mind Control. So far we've only been told that one of those abilities, Feedback, is returning.
Things may be quite a bit different than the original StarCraft, but Feedback was only useful in certain situations, and required micromanagement. I have nothing against micro, and am even a fan of it, but the ability that should be really coming back is Mind Control.
Mind Control gave the player near unlimited possibilities. You could merely take over a few units here and there, throwing off enemy raids and expanding on your own forces, or take a chance and skillfully nab one of your enemies workers and start building up a second army.
There were both advantages and disadvantages to this: you gained double unit cap, but you also had to spend a significant amount of resources to be able to make good use of this. All of a sudden you had access to another tech tree, and you could mix your forces even more effectively. It wasn't impossible to beat someone with these two armies, but it was certainly a challenge that many people accepted, and probably enjoyed.
I'm not saying that the game needs Mind Control, or that it wouldn't be StarCraft without MC, but I'm certain it would be appreciated by many, many players, and would add a fun aspect to a game that already looks excellent.