I didn't think I was into the pewpew games but I can't stop playing this one. I'm not even all that good ... my aim is strafingly bad. It turns out I love this one. I keep playing.
I think the key is the tech trees. You get XPs for shooting the other guys (and TWICE as many for healing teammates) and every few dings you can afford to buy another skill.
There are several ways to go with skills. I started out favoring machine guns, and worked up to the biggest baddest one. That worked okay. But then I respec'd for assault rifles and teched-up to get the 4x optical scope, which I now totally prefer. Respecs are available every dozen or so levels, so you can try a new stuff as soon as you're bored with your last choice.
I always first get the healing and resuscitation skills -- that's how I level up. Whether spraying machine gun fire, or controlling bursts of rifle fire, my strength is in the heals.
Now my friends need to pick up a copy. A clan on headsets is a formidable force.