Grinding is its biggest issue. It's just a total skinner box. For example... In PvP and PvE you can earn "Marks", a special currency that can be used to buy shit from special PvE and PvP vendors selling arms and armor. You earn marks by doing activities in PvE, or playing PvP... The thing is, at least in PvP, you only earn 2 marks if you lose, or 3 if you win. I recently bought a rifle using PvP marks. How much did it cost? 150 Marks, that's a fucking lot of PvP. Oh yeah, and there's a cap of how much you can earn per week, so if you start the week off with zero, and earn the max (which is like 200 or something), you'll only have enough for one item from the vendors per week from each PvP and PvE.
It doesn't stop there though, you know how I said that it was worse when I picked it up again later? Before, you just needed marks to buy stuff from these vendors, but now you have to earn a bunch of experience and level up the PvE or PvP to earn a special "Commendation" point. So you need to grind for these marks, and you need to grind to level up the PvE or PvP before you can buy stuff from these vendors.
The other thing is the RNG (Random Number Generator). A lot of what you do has a chance to give you a rare item, but like any other games with loot mechanics, it's totally random. So, at the end of PvP matches you have a chance of getting something good, the thing is aside from earning one less mark if you lose, there's no consequence to losing in PvP, you can lose horribly and still get a very rare weapon, or armor piece. So you just keep playing and playing, hoping that maybe you'll get something good. That's how they get you addicted, the chance of earning something good when you play, that's the skinner box.
Repetitive. The game is repetitive as balls. You retrace footsteps a lot, a lot of levels will bring you back to places you've been to before, and all the major bosses are just bigger and tougher versions of common enemies. Even the final boss of the game is just you fighting a boss you fought earlier in the game, just three times in a row, and even then that boss is just a copy paste of a common enemy.
There's basically no story. There's the foundation for a story. In the pre release stuff it really seemed like they had something cool going on, but at some point the writer for the game (Joseph Staten, who also did the writing for Halo) left, and it seems like after he left they just canned the story and started over from scratch. So you go into the game and it tells you very, very little. There's little collectibles you can pick up which have lore (which can only be fucking viewed online, I might add), but it's still not a whole lot.
What does the game get right? The gameplay is solid like Bungie's past games, the art and visuals are awesome, it has some great music, it has an all-star cast (wasted talent, unfortunately, as pretty much all the good voice actors just voice merchants with random bits of dialogue). The PvP is pretty fun, for awhile.
So unless you wanna grind more than teens at a rave, it's just worth avoiding.