They took mid level or nearly forgotten actors and charged up their careers.
Just the first few films had stars .. Especially the villains (like Doom is).
The Dude - Jeff Bridges
Hulk - Ed Norton (Fight Club)
Odin - Hopkins (Silence, Bounty, Joe Black)
Freida - Russo (Leading Lady)
Jane - Portman (Star since childhood)
Betty - Tyler (LOTR, Armageddon)
Blondsky - Roth (Pulp Fiction)
Fury - SLJ (Pulp Fiction, Star Wars)
Heimdall - Elba (Rising Star)
Red Skull - Weaving (Matrix, LOTR)
Phillips - Tommy Lee Jones (Men in Black, LOTR)
Widow - Johansson (Rising Star)
Killian - Pierce (Rising Star)
Rhodes - Cheadle (Hotel Rwanda, Crash)
then later
Del Toro
Glenn Close
Djimon Hounsou
Vin Diesel
James Spader
Benedict Cumberbatch
Cate Blanchett
Jeff Goldblum