What matters to me most are A.I., tech trees, variety amongst factions and art direction. If the A.I. is dumb as a rock, the game gets tedious and frustrating very quickly. The few RTS games with smart A.I. are a joy to behold as your units actually do what you tell them to and find their way across the map and kick butt. Confusing, cumbersome and needlessly slow to develop tech trees are one of my biggest pet peeves in RTS games and one of the main reasons I quit playing the RTS games that are bad. You spend a lot of time staring at the same map full of terrain, buildings, units etc. in an RTS so if you want me to be impressed with your RTS, it had better be worth staring at for hours at a time. Also, each faction should be unique enough that you can tell who is on which side without having to resort to having every unit be a matching color for each side or worse yet, having those hard to read little lables floating over their heads. The best RTS games also manage to keep the game balanced while giving each faction advantages and disadvantages that make every faction worth playing as. When only one faction is any fun to play as, that's lazy programming. Finally, the map, structures and units don't need to be rendered with the very latest in 3-D graphics for sure. In fact, I rather prefer 2-D RTS games anyway. I'll take a game with "dated" graphics that plays smoothly with no glitches on my PC or console over a game with "mind blowing" 3-D graphics that has chugging frame rates and hicups any day. But they should at least be done with some attention to detail and be pleasing to the eye. Muddy textures, over-use of the same end of a color pallet (please stop making games that are mostly grey and brown), characters who mostly look the same, plants that look more fake than plastic trees on a model train set etc. are not very nice to look at.
