Three of the four players added in the midseason of 2012 and that should be expected to advance in their own ability and their ability to help the Seattle Sounders in 2013 are already profiled. The last will not be for some time. If you look at the best cases put forward for Steve Zakuani and Mario Martinez you see that there is a chance of adding many goals to the squad. Those goals would come from the relatively impotent (when compared to the 2011 and 2010 Sounders) midfield. Hahnemann can ease the stresses that young and inexperienced keepers placed on the squad in 2012 as well.
It isn't flashy, and it isn't unique to the Sounders. Other teams added in the midseason as well. Most notably were the Columbus Crew and Montreal Impact. Those two got better, but not quite good enough. The Houston Dynamo also made significant midseason upgrades, but got enough from them quickly.
All are in the East.
Here in the West the LA Galaxy added Wilhelmsson and then lost him. Vancouver shifted their 18 at a near constant rate. They may get the same benefit that the Sounders do from actually training together.
But, in Seattle where the cap is notoriously tight the addition of a DP, a two-time Olympian, a former #1 pick and the second greatest keeper to ever wear the kit could mean as much to the success of 2013 as the potential Winter Transfer Window moves.