People often tout DKP as one of the fairer ways to distribute loot in a guild/core. The idea is simple: the more you participate in raids, the greater your ability to obtain gear from said raids via the currency (DKP) you earn.

However, this assumes that a person is able to participate in raids at all. On Icecrown, DKP seems like the most common loot system used (as opposed to the more popular loot council on Lordaeron). In my experience in several guilds, there are usually more willing raiders than there are available raids, which presents a problem: DKP becomes more of a measure of how good one is at joining raids rather than actually participating in them. Here's a hypothetical scenario to illustrate:

1. Guild ABC runs 2 raids a week, let's call them Mon raid and Fri raid respectively.
2. Raider X wants to participate in both raids and signs-up/shows up accordingly.
3. Raider Y also wants to participate in both raids and signs-up/shows up. However, Raider Y is friends with the raid leader.
4. Both Raider X and Raider Y are equally geared and have the same spec and skill levels.
5. On the day of either raids, the raid leader picks Raider Y for the raid, therefore Raider Y would have obtained DKP and Raider X does not.
6. Raider X is only able to join raids whenever Raider Y is absent. However, Raider X would have significantly fallen behind on DKP and is outbid on most gear.

I could even go further in the above scenario. Raider Y could actually be worse in terms of skill and gear compared to Raider X and STILL be selected to raid over Raider X as the spot is ultimately decided by the raid leader.

Now you could argue --- this is no different from other loot systems. If you don't get to raid, you are not getting gear anyway. While I generally agree let's compare the above scenario with other popular loot systems:

1. Loot council -- Loot council may or may not award loot to Raider X in the few raids they manage to participate in. Outcome really depends on how fair/corrupt the council is.
2. Roll MS/OS -- Raider X has an equal opportunity to obtain loot against any other raider who wants the same loot as outcome depends entirely to chance.
3. GDKP -- Raider X may be able to obtain loot if they've farmed enough gold to outbid other raiders.
4. Soft res -- Raider X has a good chance of obtaining loot if the ones they prioritizes drops.

In a way, you could (but not necessarily) say the DKP system could easily result in an oligarchical/cronyistic environment whereby those favored by the GMs/RLs decide who gets to accumulate DKP and therefore gear and who doesn't. Worse still, compared to other loot systems aside from loot council, those who aren't favored have almost no chance to actually get gear whenever they do get to participate, essentially raiding for "free" to the benefit of a select few.

Nevertheless, I don't think DKP should be done away with. There are probably ways to fix or mitigate the issue outlined above though it would require guilds who employ this loot system to be very systematic and organized.