A large factor in whether I play or don't play a Star Trek MMORPG is going to be the level of attention given to the natural objects in space in the game universe. Will they be actual objects that I can explore at my leisure? Or will they be mere graphical wallpaper, and the only objects in space that can affect our ship will be rocks and missiles?
One of the things that bugs me most about natural space objects in EVE Online is that so much of it is just eye candy. You can "mine" asteroids (i.e., have your ship automatically fire a laser at a rock while you twiddle your thumbs), and there's an Archeology skill (which apparently is just a way to generate combat missions). Otherwise, things like nebulae are just there to be pretty while you kill stuff. El yawno.
But the Star Trek experience is full of interacting with objects! Heck, how many stories have there been about a ship interacting with some hitherto unknown and potentially dangerous natural object, for which all of the ingenuity, technological competence, and bold leadership of Starfleet personnel were needed to avert disaster?
"Fully explorable" to me means that players will be able to use the many amazing devices of a starship to observe, classify, test, sample, ignite, avoid igniting, and defend the ship from all manner of strange space-based stuff. That does not mean I want to see nebulae clogging up space no matter where I go; it means that places that reasonably should/might have space-stuff will have plenty of such stuff, and that players will be able to use their ship's many systems to interact with that stuff in multiple ways for meaningful gameplay effects.
I hope Star Trek Online will be designed to be "fully explorable" in that sense.