The problem with the game is 3-fold.
1) Lack of desireable items being sold in the cash-store. Defiance's store primarily consists of only two things, fancy costumes and xp-bonus. Fancy costumes are nice, but they simply don't sell. Just ask Star Trek Online's devs, they've totally stopped putting up new uniforms because very few players are buying. Now, if they put some cool and unique weapons, that might keep them afloat for some time.
2) Features that kill social interaction and community building. Part of this is due to having no social hub for players to gather and mingle. Part of this is simply bad UI assumptions; they assumed everyone will be using voice chat and made the text chat UI totally invisible until you wanted to chat which is a really bad idea. The end result is a player base who totally don't interact, don't make new friends and don't keep each other playing the game.
3) Lack of content. Admittedly, this is the same with almost all newly launched MMOs and Defiance has actually filled the world with enough pre-endgame content. Its the end-game that's currently lacking although this week's plague sieges does alleviate this somewhat. Also, players assumed that there will be a lot more tie-ins with the show itself and were greatly disappointed when only half the episodes had tie-ins with the show and the tie-ins were a measly 5-minute "find data recorders" type missions.
If they can improve on these three aspects and if the tv show continues beyond season 2, I think there's still a good chance for the game to be successful.