When it comes to choosing gear, tanks and healers seem to differ from DPS in a significant way. Tanks and healers are trying to reach a balance between two points, while DPS is only intent on maximizing one point.
Tanks need to choose between Survivability and Threat. There's no need to put out 50% more threat than the nearest DPS, and there's no need to have so much health that you never drop below 75% health. Similarly, healers need to choose between Healing Throughput and Mana Longevity. You need to put out enough healing to keep people alive, but you also need enough mana to last the entire fight.
DPS, on the other hand, really optimizes for only one thing, DPS. When looking at gear, pretty much the only thing that matters is which gear results in higher damage.
Part of the reason for the difference is that, past a certain point, Threat, Survivability, Healing Throughput and Mana Longevity stops being useful. But higher DPS is *always* useful. There's no tension, no balance you need to maintain as DPS.
The ramifications for this show up in a lot of different places. Tank and Healer builds often make trade-offs. DPS builds, on the other hand, are pretty much the maximum damage builds possible. Tanks and Healers are more likely to use different colour gems because each color enhances a different aspect. On the other hand, DPS absolutely hate blue gems, because blue gems rarely contribute to increasing damage.
I'm not really sure which is a better system. DPS is simpler and straightforward, and makes gear evaluation a lot easier. As well, you can minimize the amount of gear you carry. Tank and healer gear can get more complex, and you can try for a balance that is very specific to the needs of a specific fight.
The DPS class that comes the closest to needing a balance are mages, and there is a lot of complaints among mages for needing that balance. Mages complain that Spirit is useless, about needing to use Mage Armor over Molten Armor, etc. Mage builds rarely emphasize the trade-off, and usually sacrifice longevity for DPS.