Their casting is absolutely phenomenal, and really adds so much to these movies. Also, they were smart to go with the Ultimates versions. Cap and Tony in particular got quite a boost in their Ultimates makeover. As soon as I saw they had cast RDJ as Stark, I started to get cautiously optimistic that these movies were being made by someone that loves the source material.
It's not even just the main characters. Blanchett, Hopkins, Close, Redford, Elba, Rooker, Goldblum, etc. These movies nail the casting from top to bottom. I don't think I've ever had this level of confidence in a studio.
IW was such a triumph. Everything my 8 year old self wanted, AND everything my adult self wanted, in the same movie?!!