I thought you'd save Kaiden! Glad you really enjoyed Virmire; I'd say it is the best set-piece mission of the entire game, and one of the best missions in the trilogy. That sense of urgency really drives the narrative, culminating in that decision. Doing it third as opposed to fourth wouldn't do it any justice, to be honest.

There are a handful of side-quests on the Citadel, but you're otherwise on rails. Mercifully, not for a terribly long period of time, but still.