The only addition I'd make is that for 5-mans, try any of the decent specs and don't worry too much about your performance and focus on playstyle and getting the feel of how to play it. I agree, DW frost isn't great for 5-mans, but as long as you have one or two geared players in your group it won't matter anyway. Does it really matter if you can 'only' do 3k dps in a 5-man in DW frost when you could do 3.5k as 2h Blood or Unholy?
In raids, DW frost, 2h blood, and 2h unholy (oblit nerf inc, blue trying to 'rebuff' scourge strike) and DC spam unholy all perform reasonably well depending on the fight as mentioned above. So I'd recommend picking one of the four main specs - the one that appeals the most - and running with it for a while.

Have fun!