i think it rates up there with the best MMO's out. sure the graphics aren't up to part with games such as EVE, but stunning graphics aren't what make an MMO great.
Despite the ever present problems we have (in game and on the forums) the best thing about discovery is the community. it has recently gotten to become a rather large community while also keeping that relatively small feel to it that i enjoy very much.
with the game being free to play it allows for casual play where as with MMO's i feel obligated to play them since i'm forking out $15 a month for it.
what makes Discovery unique is the roleplay. every MMO i have ever played was focused on getting to the max level and getting the very best equipment. never did i see anyone actually roleplaying their character. sure Discovery has it's "chat rooms" but they are nothing to the MMO's where a MMORPG has very little if any resemblance of RP in it. if RP is attempted on a MMO, the attemptee is scoffed at and made to look the fool.