A bottom bracket tool and a shimano/sram cassette lockring tool (in addition to the chainwhip) are both worthwhile investments (they pay for themselves in one or two uses). Cable housing cutters, as well (just because they weren't mentioned), and a solid workstand of some sort is important.
Getting derailers adjusted will be a learning experience- I suggest buying an extra cable or two, just in case, so you have spares on-hand.
As for things to watch out for- just make sure you thread the BB straight and in the right direction (and grease it), and when assembling the cassette, make sure everything is seated right before you go cranking the lockring on.