RTS games are often hard to get into. Advanced players credit their success to their 'uber micro' (micromanagement) skills; ie, clicking extremely fast and accurately. While that's certainly an impressive skill, a game which relies too much on micro often seems simply annoying and hard work to play. The original RTS, Dune 2, required you to move every unit individually, without waypoints. Not even the most emo of emo kids would inflict that kind of pain on themselves voluntarily nowadays.
More recent games take some of the micro out with groups. Moving a group of 12 units at once makes focus fire, light armor tactics etc viable. Special abilities aren't touched, however. StarCraft Science Vessels have no default attack, so when you tell them (as part of a group) to attack, they simply bob gently above the opposing forces without even activating a shield.
This leads to the idea of a squad - a group of units, produced together, with alloted roles. Put a medic in a marine group and when the group is told to attack, he will heal marines instead of trying to tase firebats. This frees players from rote tactical micro to make economic and strategic plays, and significantly enhances the value of these units. Another example - put mages with footmen and they auto-sheep the enemy's archers.
It also opens up a whole meta-game of squad design and trading, reminiscent of collectible card games. In effect it layers persistent, solo and social capabilities onto the RTS meat, helping turn it from a one-shot experience only the hardcore will truly explore into something more gripping. WoW has shown that simple base-level play with tangible advancement helps draw a mass audience. Spore is taking a page out of Facebook, it's time for RTS to join the club.
Bonus cow level: imagine an RTS with a window manager, on a multitouch screen. With cows.