-Let us change acts in one game. In D2, once we completed a quest or got to a new act, we were able to freely jump from one act to another in the same game. This was especially good for mfing (sorry I hate the term 'farming') because we could kill several bosses from each act without having to repeat the quests. I think Diablo 3 should be the same way. The current system of creating games by quest and being forced to repeat the entire quest(s) / being bound to one act until you kill the act boss is way too linear and repetitive, and it makes MF runs take way too long. And on that note, I think we should retain our valor stacks when we do change acts.
-Let movement speed spawn on other rare items besides boots and remove the cap. I understand the idea behind only letting certain stats spawn on legendaries, but I think movement speed should at least be spawnable on rare pants and chest armor. Running around with 12% movement speed just seems really slow, especially compared to Diablo 2 where faster run/walk could spawn on pretty much every item and there was no cap (not to mention all characters could teleport). Since only Wizards can teleport (and there's a huge cooldown on the skill), it seems appropriate to allow us to gear up with tons of movement speed.
-I think the create/join games system should be changed to be like how it was in Diablo 2. Instead of joining games by quest, we should be able to create/name our own games. This system worked fine in Diablo 2 and made it a lot easier to socialize/trade. This wouldn't even require much of a UI change; just make it so the "public games" tabs shows a list of all available public games (just like in D2) instead of quests.
-Allow 8 players per game instead of 4 like in Diablo 2. It's just more fun that way, especially when it comes to pvp. Having the option to 1v1, 2v2, 3v3 etc. or quest in a party of 8 was part of what made Diablo 2 so fun.
-Add more socketables (jewels, runes) and bring runewords back. White items are completely useless in this game and runewords would solve that problem completely. Allowing the blacksmith to enchant/imbue white items like Charsi did in D2 would be another cool addition. This could be implemented by adding a rare blacksmith plan that teaches it how to imbue an item, a side quest that gives the blacksmith the ability like D2, or simply by allowing the blacksmith to do it automatically.
-Give us the option to add sockets to items. That's another thing Diablo 2 had that I don't think should have been removed. This can be done the same way as the enchantment, except using the jeweler instead of the blacksmith.
-Let valor stacks/mf affect drops from unique monsters. 90% of the time I see a unique monster pop up (including Izual), it drops nothing but a few white and blue items, regardless of my 5 valor stacks or total mf %. In my opinion, they should be treated just like bosses or elites and have a guaranteed extra rare (or 2) drop if you have 5 stacks. Giving them an increased chance to drop legendary items would be beneficial as well.
-Add more content. I'm sure there's going to be another act or 2 either in a patch or with an expansion, but the game really needs some more content. Throw in some hidden dungeons and some side quests. Put minibosses at the end of the extra dungeons that have different abilities than the other bosses/elites in the game. The random special events and caves throughout the acts are cool, but there's very few of them and there isn't much incentive to explore any of those places or do any of the events because they usually aren't very rewarding.
Feel free to comment on any of these suggestions or add something to the list.