Eh, Cap was serious in The Avengers and everyone found him boring and bland. I love Cap but I can admit that. He actually had some personality this time around and I thought it was a big improvement over TA. Every other Avenger is funny/witty. Obviously Stark, Ultron, Banner, Thor, Widow, and even Hawkeye.
Making Cap as serious and no-nonsense as he was in TWS would make him stand out like a sore thumb in the much more fun and lighthearted AoU. He'd come off as the serious, can't-take-a-joke guy that no one likes. This is an ensemble movie and when everyone is entertaining, Cap has to be too. And there are limited lines for everyone. Not to mention that Cap is in a totally different place in TWS. He's lost, lonely, doesn't trust anyone, is in a situation he doesn't feel comfortable in, etc. In AoU, he's finally found his place in the world, saving the world among friends. Makes sense he'd loosen up.
Both Cap and Thor are serious warriors who were both more fun and lighthearted this time around. I know Hemsworth even asked for it. I thought the humor was very well done with them as both were true to character. Thor with his ridiculous, over-the-top Norse speak and Cap with his goody two-shoes "language!" gag.
Sure, Whedon went a bit overboard with Cap in the final battle, but that's what you get with Whedon and it's not that bad. And it's not like Cap wasn't Cap anymore, he had plenty of moments where he showed his TWS side. In the aftermath of Ultron's attack in the HQ, talking to Stark at the farm, his monologue/battle strategy before the big battle, etc.
Cap was Cap, just a bit looser. I'm the first guy who hates too much humor, but I didn't have a problem with Cap in this movie. These guys can be serious in their solos especially with Civil War and Ragnarok coming up, but the Avengers are fun, entertaining event movies and I'm glad Whedon embraced the fun aspect. The majority seemed to love Cap a lot more than in TA, so he made the right choice. I adamantly disagree that Cap should have been as serious as he was in TWS, or that he can't be funny. Have you guys completely forgotten TFA? Steve was always the adorable dork.