I enjoy the occasional puzzles thrown in here and there. It adds a bit more reality as opposed to the "run and gun" method. Hefting barrels and boxes in order to "activate" the appropriate mechanism can be a nice little challenge.

I also like the way the game catalogues your quick save progress. Now I don't have to worry about pressing the quicksave button only to discover that a missile was about to obliterate me and the most recent "official save" was 2 levels ago.