believe that it boils down to this, personally.
Gameplay: This encompasses everything, campaign, skirmishes, and mmorts mode. It is of utmost importance that this is gotten right. Otherwise it has to fall back on modding and map making which wouldn't be as great as having just plain great gameplay.
Graphics and Physics: This is rather important. You want to get physics right and have good graphics, but they aren't paramount. Great games are boosted by great graphics. However, great games can still have bad graphics. A large number of people still play Starcraft even though its graphics are quite outdated.
Modding, Community, Custom Usermade Maps: This game boasts a strong editor, and I hope that it is true. This category has kept Warcraft 3 much more alive than the traditional gameplay possibly could. New, innovative maps can add greatly to the fun had in this game, and this category will also attract people mostly there for good editors and the like. (Also, I have a question, would it be possible to have a modded mmorts (at least if they have a dedicated server or whatver and can do that)).
Story and Immersion: This category is rather big to those who like to play campaigns. It would be a big boon to the game if they get this right in the campaign. Quests in MMORTS mode are also a large part of this. They should have a small sort of meaning, even the lowliest of quests, to the overall scheme of the world and its progress (It could be very very very small). Quests having lore value would be great to have (it would also be nice to be able to have some sort of roleplaying elements in the game, I would like to RP a little bit).
Sounds and Voice acting: The sound part is important, and so is the voice acting. The voice acting isn't that important, but it detracts from the gameplay if it's terrible. Sounds are more important because you would hear them more often, (many of them at least) and if a sword hitting someone sounded like a dieing rabbit, I would honestly start to get annoyed very much (unless it was kinda funny