I'm a casual player, and I just reached CMDR. I did it mostly from Quin missions and Sulu's defend missions. It was fun and there was a good mix of things to do as far as ground and space combat with a little exploration mixed in. One thing that bothers me though, is the lack of instant gratification. I spent a good 45 minutes doing a few missions where I only got that xp boost at the very end.
I remember taking a statistics class back in college and we studied the effects of slot machines and how that the instant gratifiication of all those lights and sounds and coins made players happy and made them want to stay. I hate slot machines, but I do want some rewards for what I am doing in a game.
Maybe they could give a slight xp boost for finishing parts of missions, or give some xp for killing monsters.
Or maybe they could make a battlfield for more pve experiences. Just a thought. I'd like to log in for 20 minutes every now and then and get some xp.