Perfect Achievement/Exploration/Unlockables System.

I made most of my comments about the game in a reply below, but I do want to mention one extra thing I forgot to say, the Riddler crap was perfectly done. It was totally optional, but you got awarded exp for it, which motivated you to do it (at least early on before you had all the upgrades) plus they put in maps that were all fairly easy to find but you had to find one for each area. This was brilliant. By doing this you got approximate locations to the "secrets" and "riddles" scattered around, which made it much more fun to find them.

Most games that include a system like this end up with you having to jump between the internet and the game looking up the location of that "last thing I need" or "how the hell does this totally unintuitive puzzle work?!" but being able to open my map and tell for certain that I was in approximately the right area, looking in approximately the right direction, encouraged and allowed me to solve every single side-quest secret and unlockable without any outside help. That was really well done and I was really happy with that design decision. This is the only game where I have ever continued to play after the main story was over just to complete all the extras and the reason was simple, the map made it FUN and ENTERTAINING to do so, as opposed to tedious and time consuming.