There's a little reported bug with the GOTY version on steam that causes some DX11 point lights to not blend correctly and show their polygonal edges.
Here's the steam thread with the fix:
And the fix itself in a google drive link: https://drive.google.com/file/d/0B60dyrffWO_PNU50N0h3bE9fZkk/view?usp=sharing
Visually the issue is very noticeable when looking at Tyger helicopter searchlights and the first room in the museum with the TVs pointed at Batman, they look like giant polygonal shapes that brighten anything within or behind them from your POV.
The fix replaces two shader files that were sourced from the original GFWL release, the steam version's ones are the culprit. If someone could archive these and add a link and description on the wiki page that would be great.
Unrelated, but I was trying to post this on the arkham city discussion page but captcha kept on resetting after pressing add topic, sorry I just made the account to try to put it on there so if I did something wrong my mistake.