9. Multiple Playable Characters Were Underused
Although it was teased by having multiple playable characters in the challenge rooms in Arkham City, as well as including Catwoman as a controllable protagonist throughout the story, Batman teaming up with sidekicks to fight crime wasn't properly included until the final game in the series.
Fortunately, while the scant few moments where players were allowed to control two heroes in a fight and perform epic dual-takedowns were suitably satisfying, these scenarios appeared only a couple of times during the campaign, and were often in rather restrictive set-pieces.
You were never allowed to swap between characters in the actual open world (something which was present in City), and after so much hype, the restrictions of these character-switching sections made them less enjoyable than they should have been.
Obviously it was still great to see Batman take on goons toe to toe with the likes of Tim Drake's Robin and Dick Grayson's Nightwing, but after such a long wait, it was disappointing that they weren't a larger part of the story.