Activation, can't really buy anything for it. Apple's activation scheme is one of the best (maybe THE best as Steam is theoretically better, but seems to redownload THE ENTIRE GAME every time you move it to a new computer, even if you have a local copy), but still. I *THINK* Apple even lets you deactivate a copy of iTunes that you no longer have access too (useful if a hard drive dies or the like)...I think, I'm not totally clear on that part. So at least theoretically that makes them one of if not the best, but...
I'll confess I have caved and bought ID's two RPGs thoug, as I loved Orcs and Ogres or whatever it's called on the DS (though it's a lot more fun and controls 100x better than Wolfenstien RPG at last).