Even seemingly 'open-ended' games promise freedom whilst at the same time prohibiting expression. Oblivion succeeded in providing a character creation/evolution framework that was highly granular whilst not being stupidly complex. That was a big achievement. Its world was (at least on release) mind-bogglingly big, luciously detailed and free to roam at any given time. Another plus. However, what let me down was how one dimensional and 'lifeless' quests and interactions with NPCs were. Maybe no more so than every other game up to that point, but still, I had expected a little more.
Once you've played the speechcraft game for the seven-hundred-and-fiftieth-time, you start to understand that the 'role playing' aspect of Oblivion is more 'choose your race, equipment & demeanour, collect your role on the way out'. Oblivion's highly linear quest/NPC interaction structure was pretty much the only letdown for me - but still a major one, as it neon lit the hoops the game provided.
IMO, the inflexibility of storyworld information and character is one of the last major hurdles games need to overcome (particularly RPGs, which would likely benefit most from more dynamic, generative structures for their narrative content). To this end, I'm currently pursuing a PhD on related issues. Will let you know if we manage to crack it. Don't hold your breath, though...
Also, though CoD4's story mode succeeded in being compelling and throughly linear, this is completely counter-balanced by the online multiplayer. CoD4's multiplayer modes take done-to-death formats, trim the excess fat, add some muscle and adrenaline, and comes out with that magic game quality Matt talks about - total freedom of expression within well-defined gameplay limits. Right down to the custom character classing, CoD4 nails it.
You have to admit, if CoD4 was just the single player modes (which are great, don't get me wrong), you'd struggle to see how you'd got your money's worth.