Hindsight is a wonderful thing; it allows people to torture themselves endlessly over past mistakes, and is basically one of the pillars of evolution - meaning you learn from your mistakes even as they're making you cringe horribly. The same goes for film-making.
Marvel have had a mostly smooth run with the MCU, despite a few major bumps along the way. If they could do it all over again theyd surely tweak a few things like The Infinity Gauntlet making a cameo in Thor, or casting Mark Ruffalo in The Incredible Hulk and avoiding the Ed Norton fiasco entirely.
Norton isnt the only mistake theyve made, of course, but more specific has been the studio's frustrating knack for casting great actors for entirely the wrong role. In some cases this is despite the obvious case for them playing a different Marvel character altogether: sometimes, they just don't read the signs.
Obviously fans are quick to point out to Marvel and luckily hindsight affords us the opportunity to right those casting wrongs and put Marvel's square pegs in the right holes...