I love playing SC2, I've been playing Starcraft since the original came out so long ago. My problem is that I find it cumbersome to play. I can never get out of Bronze League save for people instant quitting on me sending me temporarily into Silver, so I'd say I'm a less than average player. My one single thing that prevents me from doing better in game is that I just can't manage every aspect of the game in real time on faster speed. Que the workers, build the buildings, scout, make army units, fend off rushes or drops, spread the creep, check the minimap, tech up, get upgrades, attack with army, build new units to replace the dead, don't forget to use abilities, expand while attacking, it's just too much to do all at once. When I play I feel like it's a frantic rush to try to manage every part of the game at the same time, I battle the micro and macro more than I do my opponent. It's why I play more versus the AI, I can take time to breathe and still have a chance of actually winning a match now and then.
I know a lot of people love how APM demanding the game is, but it hurts the game a lot IMHO with the mass market. Skill without APM is harshly punished, you need both to win competitively, but even high APM can win you games without much skill. Starcraft is very much a "twitch play" favoring game.
I wish Starcraft was more macro and strategy focused rather than micro and APM focused, and I feel like it would appeal more to casuals if it was. I think the Dawn of War games balanced this much better, but they are nowhere near as popular as SC2. Kohan is my favorite RTS of all time because it focused so heavily on tactics and strategy and was very lenient on the APM side. There really aren't any modern RTS games that break the SC2 mold, not today anyway, certainly none with any substantial playerbase.
Starcraft, by design, just isn't a casual friendly game to play. It's designed for the APM monsters to excel at.