There's no cover button - indeed the game generally seems to regard cover as for wusses.
A 40k Space Marine doesn't need to "take" cover; he's already wearing it.
I was thoroughly unimpressed with Starcraft 2, because the developers seem almost hell-bent on refusing to innovate. If you really want to see something amazing and you're pointing your eyes at the RTS industry, be sure to take a look at the mod developers, because they've done far more impressive work.
Why does new stuff always have to "innovate" to be good? Even if SC2 did bring a whole lot of "innovations", what good would it be in the long run? People that always want "new and shiny" would abandon it the moment something newer and shinier hits the market, because what was new and shiny at SC2's release is now old and boring in comparison. That is not the audience SC2 is made for.
What Blizzard wanted to do with SC2, and I think they did a good job, is aim for the long run by taking elements that are known to work and recombining them. SC1 was (and still is) played for what now? More than twelve years? That success doesn't come from "new and shiny" because "new and shiny" has an expiration date. Some factors of what make up the success of SC1 are the "easy to learn, hard to master"-principle combined with probably the best balance in any RTS out there, which are the fundamentals of competitive play, and observer friendliness. SC2 was made to follow up on that success without a complete overhaul of the fundamentals.
//GO.SYSIN DD *, DOODAH, DOODAH