Customization - very important. Character progression - not so much. This is a skill game, not a classic MMO-style game where as you level you get "better". Progression should be limited to unlocking certain wargear or maybe even a couple abilities/perks. Otherwise, skill should determine power.
I disagree entirely. Depth of PvP gameplay will keep people coming back. The last thing I want is an element of grinding in this game. People aren't still playing TF2 because of it's character progression. New customization options, and different gameplay options within the classes gives plenty of diversity. Enjoyment of PvP and the slow release of new content keeps you coming back. Levels in of themselves are meaningless, and just create a barrier for entry into PvP. In a PvP focused game you can either have shallow level progression where people cap out fairly quickly, which largely defeats the point, or have 'advancement' come in the form of customization where new equipment gives you new means of playing your character but doesn't significantly affect the overall power level.
Aesthetic customisation is extremely important to me. Progression; not so much, so long as there is a variety of wargear to mix and match. eg: bolt pistols vs plasma pistols, chainswords vs power swords, and the like. Being able to pimp out my main character to show my dedication to my faction because I accomplished something worthy of reward or because I dumped a buttload of money on store-exclusive customisation options is something I really want from this game.
Progression, skills, talents and gearing should be very diverse. So you can play your class in many ways. Be personal, like people. Some just hit better and some just can take more beating others rally their mates better. You know what i mean.
No vertical progression except thanks to elite classes you need to sustain reputation to deploy (maybe in limited number in an area) as to gather enough of another resource... like requisition maybe, some loadouts customization could be nice but without the slightiest raising in power. Heavy cosmetic customization limited only by an Inquisition like rigor for chapters/warbands/klans/craftworlds colours and schemes autistic geek respect. EDIT : which means : no angry marines, no rainbow marines, no my little poney Eldar, and the such...
I wanna fell like im actually serving the BLOOD GOD KHORNE! Customization is a big deal to every one I believe.
There should be some vertical progression too. Leaders should be in some way better than regular footmen. Not in a too overpowered way. Maybe some mutual passive abilities got boost from 5% to 10%. And chance to get hands on better gear.
