The 'Open World' environment has me the most excited. The thought of me and three friends splitting up into two groups, scouting the city, searching for clues, collecting supplies, perhaps establishing a 'base' somewhere in the city by bringing found vehicles and weapons back to that location.
It's just an exciting idea to think that it'll have an essence of 'survival' as you're organising yourselves and heading out into new locations, it just makes me wish they didn't abandon the ODST's ability to use equipment.