You basically have the following styles to chose from with regards to headphone backing types.
Open
Semi-Open
Closed
True open headphones are open to all sound. They isolate nothing and if loud enough anyone around can hear what is being played on them. Of course they need to be fairly loud as the direction of the sound is still aimed at your ears and not the outside world, but they can still be heard by others.
Semi-Open block some of the sound going in an out.
Now as far as sound signature... that is again personal preference. Loud, and certainly bass that rumbles on and on, can drown out sound from other frequencies happening at the same time. Which means in a game the fainter noises, like footsteps, could be washed out from load bass explosions. Big bassy explosions are very nice, fulfilling, and make you go DAAYYAAMMM!! when they go off, but they can also lose you a bit of competitive advantage as well. Which is why some gamers go for headphones with as much clarity in the mids that they can get and don't mind a weaker bass response and roll off. It really all depends on what sort of gaming audio experience you are after.
Then again, you can be like me and have many different headphones to use based on the listening experience you are after.
Closed block most of the sound coming in and going on.
Open headphones tend to have the best soundstages, with semi-open ok soundstage, and closed headphones rarely having good soundstage.
When it comes to actual imaging, that is not really a function of the headphones per say, but the source sound and what is routing that source. However, some headphones are better at reproducing accurate imaging compared to others. All the good "audiophile" headphones, which include beyerdynamics, are going to have good imaging. For gaming, the differences are in the soundstage representation. DT770 are closed and don't have much soundstage. The 990s are "open", but really not fully open as others. Still, being open means that they are more open than the 770's at least. They will have a bigger soundstage in comparison to the 770s.
I went ahead and purchased the 990 pro 250 ohm to give it a try, comes in tomorrow from amazon. What amp/dac would you recommend at the best price? As cheap as possible!