I think it also depends what kind of game you're going for - a game for a bit of fun, or a seriously competitive game.

I used to play a lot of RTSs back in the day. I loved games like Red Alert/2, Generals, etc. I'd set up my base, build up my army, and attack. Low stress, and it was more fun to see how your units would fuck over the opponents army. I know there were people who played this a lot more competitively, but my point is it *could* be played a lot easier.

Starcraft on the other hand, when I first started playing I hated it. Campaign while cool setting/story, was a pain in the ass gameplay wise. Multiplayer? It was too stressful. And it had a steep learning curve I couldn't be bothered getting over. I used to join the No-Rush 20min games and whatnot so I could try to recreate the 'mass up army, then attack once and see who wins'.

Of course, once I started actually practicing it properly, getting over the learning curve, practicing builds, practicing micro/macro and eventually picking up the competitive aspect of it, I started to like Starcraft a lot more and now I love it more than any other RTS. But the enjoyment mainly came once I got over that hurdle.

The other thing is variety in races. Which comes in hard when you're trying to make it fair and balanced. But Warcraft III, Starcraft+2, Generals...each race was completely different to the other. Compare that to Red Alert 1, where each race was basically the same with reskinned units and maybe one or two special ones. Starcraft takes it even further where even the most basic building mechanics are different.

 

Dependent on RTS type. Personally, I enjoy the smaller scale games more than the mass war efforts.

Instead of throwing dozens of units at each other through macro, I focus on a core 20 unit squad that I micro for success.

Large scale fights might be fun for some, but I'd rather have an RTS where it's a small scale combat with micro, stealth, and response being the main factors over who can rush 100 units and throw them around at better spots.

 

I'm a major RTS fan so I might as well chip in.

1) Balance. In a lot of the RTS games I play, the fun factor fades away very fast due to balance issue. How can the game be fun when some asshole just roll out some epic/high tech unit that curbstomp every damn thing you throw at it?
2)Graphic. Surprisingly, graphic is one of the thing that can hook a person it. I can't stand SC, it hurts my eye looking for the units. Neither can I look at Red Alert 3.

I wouldn't touch on the rest since it is rather well covered by the guys before me.