WoW, like many MMOs, has three primary roles. Tanks, Healers, and DPS. Their roles are something I'd like to look into deeply later on since it's not as easy to sum up as you would expect, but for now, we'll just go with this:
Tanks decrease total incoming raid damage.
Healers prevent death.
DPS kills targets.
I played a Warlock from early Vanilla (Magmadar attempts) until late BC (Brutallus, HOOOO). I don't recall what my damage output was like against Ragnaros, Nefarian, or C'Thun, but I definitely remember dealing ~1k+ DPS in Kara and Gruul, pushing up to 2-3k DPS in Sunwell. I watched Recount and WWS, modified gear, improved, regemmed, respecced, and tried to do slightly more DPS, both overall and on each individual boss, every single week. And I did! My DPS went up. And up, and up. Recount proved it. WWS proved it. Say I did 2400 DPS one week. I get a new piece of gear, next week I'm doing 2450 DPS. Improvement! Right?
I played a tank as well, since my guild only raided 3 days a week during BC. So I ran pugs, as a prot warrior. I didn't really have a stat to track. All I knew was that if I lived, I was doing well, and if I died, either I messed up, or healers messed up, or someone messed up. I got better at picking out when healers were terrible, or when DPS was slow, or when I'd tried to strafe move and accidentally turned my back. I started tracking avoidance, and damage mitigated. If I used to avoid 70% of attacks, and now I avoid 72%, I'm living longer! If I used to block 2,500 damage, and now I'm blocking 2,600 damage, I'm living longer! Right?
My roommate plays a druid. He's the fastest druid in existence, whipping out the Tree Dash to catch up with me as I charge headlong into pack upon pack of add. We talk mechanics all the time, often about Lifebloom, efficiency, regeneration, and throughput. Tiers go up. He has more mana, which means more heals. More spirit, which means more mana per time. More Spellpower, which means each heal does more. Thus his ability to heal constantly increases. Right?
But each of these points is wrong. That's the tricky thing. You see, you can't measure someone's increases in a vacuum. There's no denying that a Sunwell geared tank will survive better against a Karazhan boss, or that a DPSer in Sunwell gear will kill Gruul faster. The question is whether a pre-Kara tank can survive in Kara longer or shorter than a pre-Sunwell tank in Sunwell.
A DPSer will always see their damage increase. But while they'll feel warm and fuzzy, thinking "Yes! I'm doing 10% more damage!" They won't realize that bosses have 20% more HP, thus boss fights are taking longer and longer. Tanks and Healers will feel the pinch more though. If your tanks HP has gone up 50%, but your healing per cast has only gone up 30%, you'll feel like your healing power has actually decreased. If a tanks HP has gone up 50%, but a boss' damage has gone up 60%, his survivability will feel lower, even though he's got significantly better gear.
Additionally, tank survivability has to be based off both incoming damage and incoming healing. This is problematic, because avoidance does not decrease tier to tier. In other words, if you used to take 50% of attacks in Tier 1, and you take 40% of attacks in Tier 2, the attacks have to hit significantly harder, and then have to be inflated by the increase in HP/S. So while before, you were hit half the time for 40% of your HP, now you're being hit a little less than half the time for 70% of your HP, increasing the strain on healers, the danger of tank spike death, etc.
Anyway, my point is that overall, DPSers will see their damage increase from tier to tier while tanks and healers won't see their survivability or actual healing potential increase from tier to tier. Even if the DPSers understanding of the change is somewhat flawed, they at least have a number to look at, saying "it got bigger!" while tanks just say "I'm dying just like I did ever so long ago" and healers say "I can't keep you up through this" followed by "Oh wait, I can."