With all due respect, Downey Jr is a good actor, but usually Nolan brings good actors and actresses, to his movies and that proves with the cast with: Michael Caine, Morgan Freeman, Gary Oldman, Christian Bale, Liam Neeson, which IMO are all strong actors and better than Downey Jr.
Haven't watched The Avengers yet, and i wasn't really looking forward to it to be honest, but since people are amazed by it, theres maybe something to look forward to, but when it comes to TDKR, i doubt Nolan would for the first time make a bad movie. Nolan's batman franchise is on another level when it comes to super hero movies.