Though it lessened after the film came out, Hank's age (and retired status) and Janet's Death by Origin Story. While the two are members of the early version of the Avengers Initiative, many fans feel this isn't enough as they still don't get any solo focus to themselves (with Janet being killed in a flashback (her only appearance) without any dialogue), and it means the two still don't get to hang out with their comic book teammates, who the two are very close to, with the focus instead going to Scott Lang (who, as seen in the comics, is regarded as less interesting than Hank). Others however feel this makes sense as Hank's iffy history makes him harder to sell (many non-comic fans going into the movie are at least aware of his reputation). Once it was released, many are at least content with Michael Douglas' performance, but regret not getting a film about Hank or Jan (and by extension, the classic Avengers line-up ever being realized on screen, without some creative developments).
The news that Edgar Wright left the project in May 2014 (after being attached since 2006). Many did not take this news particularly well. There was also a split between people supporting Wright (Marvel's major rewrite of the film was Executive Meddling that he was rightly angry at) or the studio (Wright shouldn't have been surprised that Marvel wanted to tie the film into the rest of the MCU). Wright also didn't help himself by explicitly comparing himself to Buster Keaton, and his difficulties working with the big studios. People who wanted Hank and Janet as founding members of the Avengers like in the comics also resent Wright for making the characters off limits due to his pet project, then leaving anyway, particularly Janet fans as it was confirmed by Peyton Reed that she wasn't going to have any role in the film despite this, until he got given the project and insisted on giving her backstory and a way to come in later, indicating Wright essentially blocked her use only to not use her (and make her unusable in the process).
There's been some debate over Hank deciding to destroy Pym Technologies and all of Cross' research. Justified because of the potential lethality of the Pym particles? A two-bit thief with a few weeks of martial arts training goes toe to toe with, and ultimately defeats, Falcon armed with nothing more than his fists. The Yellowjacket armor is a bajillion times more lethal and would have been mass produced (by HYDRA, though Hank didn't know that when he formulated his plans). Or not justified because Hank decides on his own not to trust Nick Fury and the Avengers (and Tony Stark in particular) with his research and recruits a career criminal to do the job?
Michael Peña's "Luis" character is starting to get some chatter too on the "Maybe Racist" front. Some, including a number of Hispanic people, feel that he's hilarious, very real and grounded, has some nice bits of heroism, and non-stereotypical views (wine tasting and appreciation of neo-cubism art), not to mention Peña having a lot of say in the character's dialog, making him just a fun character, while others, including a number of other Hispanic people, feel that being the only significant Hispanic character and existing as a comic relief sidekick criminal to the white lead and the butt of many jokes makes him, at the very least, insensitive.
Hope becoming Wasp is something fans seem to be split on. Some fans really liked Hope for her Tsundere personality and relationship with Scott and Hank, and appreciate her actress' talent and ability to play the part. Others, however, felt Hope was unfun compared the male characters and accuse it of sexism, and want to see Janet as the classic Wasp explored due to her much more entertaining and endearing personality. Some have suggested they Take a Third Option and have them both take the role (ala the Flash Family or the Hawkeyes, or Jean Grey and Rachel Summers), though.
