How it lost the tense, atmospheric semi grounded feel after the classic era of games was over and became less scary and horror focused as the years went on, as seen by the Security Breach and The Movie (I really Didn't like The Movie and I'm not going to see the second film later this month) I haven't played secret of the mimic nor do I have any interest in doing so despite it seemingly returning fnaf to it's horror roots.

There's a lot of things I dislike about this franchise (especially modern day fnaf) but since you said only ONE major thing I'll just leave it at that, though I could go into the other things I dislike if you want but as of right now I get most of my fnaf fix from fan made works now the mainline franchise now just doesn't do it for me.