In most games, durability is an advantage. Items with a high durability last longer than items with lower durability, and thus are more valuable. This makes intuitive sense.
In WoW, however, high durability is actually a disadvantage. It seems counter-intuitive, but looking at game mechanics quickly shows why. The most common form of item damage is the 10% durability loss from death. However, the cost to repair an item is based on per point of durability lost. An item with higher durability loses more points for each death, and thus costs more to repair for each death.
For example, take two items: blue boots with 50 durability, and epic boots with 100 durability. Let's say it costs 10 silver to repair one point of durability. After one death, the blue boots lose 5 points of durability, and the epic boots lose 10. It costs 50 silver to repair the blue boots, and 1 gold to repair the epic boots. Having lower durability was actually an advantage for the blue boots, which just does not seem right.
And this has a startling amount of ramifications, especially in the endgame.
First, gear with higher item levels tends to have higher durability. That means that as your gear gets better and better, your repair costs increase. Because the raid dungeons do not drop increased amounts of cash, it means that the amount of gold needed to support raiding increases as gear gets better.
This is a big problem for healers and tanks. As a character levels, the rate of gold farmed continues to increase, allowing them to keep pace with their increased costs. At 70, though, this changes.
For DPS, the rate of gold farmed continues to increase as new gear is gained, allowing the DPS to match the increase in repair costs. However, for healers and tanks, the new gear does not help increase the rate of gold farmed, meaning that they have to farm for longer periods of time to keep up with the increasing durability on their gear.
Secondly, the increasing repair costs leads to higher cost of death, which leads to more risk-averse behaviour. When a single wipe costs 5g or more, a healer or tank is less likely to engage in risky actions like joining pick-up groups. This is bad for the game as a whole, as it works best when people are actually willing to do things, to take the risk of dying.
The solution is to change the repair mechanic. Rather than repairing on a per-durability-point basis, repair costs should be done on a fixed percentage base (by level). It should always cost the same for a level 70 to repair from 0% to 100%.
To return to the above example, if costs 5 gold to repair from 0% to 100%, both the blue boots and the epic boots cost 50 silver. So for a healer or tank, it takes them the same amount of time to farm for gold to support their new gear.
(The DPS ends up with more free time, and possibly more money as a result, but I don't really see that as game-breaking. At least not as game-breaking as forcing the healers/tanks to farm for continously longer periods of time. Besides, the rogues and hunters already avoid half the wipes anyways.)
This way there is symmetry between the percentage loss when dying, and the percentage gain when repairing. Costs are now fixed, and no longer require increased farming time. Durability is once again an advantage as--not counting deaths--an item with high durability will last longer and not cost as much to repair.