5 Inherent Problems That Explain Why Every Marvel Villain Is Boring
4. The Focus On Heroes Means Villains Don't Have To Be Interesting

The MCU is a franchise built on heroes. In fact, the best reading of the series on a macro level is that it's a dissection of what makes a hero - they can come from anywhere, with any past, and any ability. In that vein, aside from Thanos (and even he'll become an afterthought come Phase 4) and arguably Loki, no villain has any real impact on the overarching narrative. Whereas Magneto is the key though-line to X-Men and the Joker was the lynchpin to The Dark Knight Trilogy's story, for Marvel the villains take a real backseat. And that's exactly how they want it.
Traditional hero properties feature one or two good guys going up against a massive rogues gallery, which meant a lot of stock had to be put in fleshing out these unique antagonists (see Batman or Spider-Man). With the shared universe of Marvel, and The Avengers in particular, it works differently; here there's a whole raft of heroes ready to be merchandised and imitated - instead of playing as a hero and their many villains, kids can now all be the hero against an army of goons - so the villains are, from a marketing standpoint, a secondary concern.
When looked at from that angle, it's no surprise Marvel have taken the ethos that the villain doesn't really matter (hence why they're always dead come the end credits). And it works - as much as the bad guys are bland and repetitive, Ronan's lack of depth or Ultron being a caricature of the comic character didn't really affect how people reacted to their respective movies.