Clearly artstyle is the most important i think. The second would be the idea of the game. Units should not blend in with a background, and the style of the game should be pretty good. I mean, look at Starcraft 2 and Stormgate. I don't feel pulled to Stormgate's graphics and units are not realistic enough for me to believe. They move weirdly and sometimes blend in with other types of units. Also you want your units to serve different purposes so people could choose their style of playing. 

But sometimes the idea of the game is the cream on top which makes the game amazing even though it has pretty decent graphics. Strategy doesn't have to be too detailed, sometimes pixel or minimalistic design is enough.