Like character advancement, the advancement engendered by loot can be seen as vertical or horizontal. An item that vertically advances your character has flat-out better stats than your previous item, but does nothing that the previous item didn't do. Horizontal loot advancement comes from new abilities granted by items, or different kinds of damage dealt, absorbed, mitigated, etc.
Vertical loot is only exciting if it represents a big gain. Horizontal loot can be more exciting more often without necessarily advancing the character, because it can afford the player more interesting options to try out as long as the metagame is not in a moribund state.
The real question is: What effects can an item have? The more you restrict this, the more boring the system will be. If you go too far, you risk imbalancing the game due to the slot machine taking over and player skill being no more than a secondary factor in gameplay. This isn't a problem in most MMOs because such games are no more than social environments with slot machines that require mostly-thought-free manual effort to pull the lever by killing mobs, opening chests, and completing quests. In a(n ostensibly) skill-based game like GA, a prevalent slot machine turns what otherwise is a fun PvP system into an awkward environment where time-based play and skill-based play clash.
The universe of possible useful pieces of loot in GA is too small. It's exacerbated by the fact that the sole way of progression available, vertical progression to higher bonuses, caps out at 21% with an exponentially lower chance of getting loot of higher qualities once you get above the base 10%-ish. Boring, linear vertical progression with no horizontal opportunities is not fun. If I know what I want and feel like I'm just waiting for the random number generator to swing my way, I'm having less fun than if there is a reasonable chance I may find something cool that I hadn't considered.
The number of useful pieces of loot that you can possibly find starts out small and only gets smaller. Because GA is skill-based, players set their skill specs in stone and know exactly what they want to make it work optimally. Because gear is primarily vertical in variety, the player knows exactly what he needs at any given time for his spec if he has even a minimal knowledge of how the game works. there is no chance of a serendipitous drop--only for drops that either give the player a "finally" feeling, or drops that are useless to the player.
You only get loot in GA when you win missions or PvP matches. In PvP, particularly, your chance of victory is largely dependent on the skill level of the rest of your team. Only the top 5% (or less) of players can carry any team to victory—and even they can’t successfully do it every game they’d like to.
Global Agenda’s loot system is a boring, naked time-sink. The best that can be said of it is that it provides an object lesson in how not to design a loot system.