- Warriors going from a famine to a feast of rage...early in expansion Warriors are rage starved, later on they can't get rid of it due to crit/haste scaling.
- In Cata especially, most healers returned to the Old Wrath model in some ways when spirit/intellect exploded out of control, resulting in near limitless mana pools. A friend of mine, a druid healer, found that despite Blizzard's claims, Rejuvenation spam became very effective later on in Cata.
- Later on in expansions PvP gear stat inflation has often lead to PvE gear becoming completely inviable.
...so long as it had the same level of secondary stats.
In WoW, all the roles (with the exception of Tanks to a degree) has that "primary" stat that is always their best for DPS. For the most part throughout Cata this has been successful...a hunter or rogue is going to take something with more Agility when presented and its effective weight is much greater than any other stat. So even a small (1-3) increase in a secondary stat could make an item an upgrade. The problem is not the main stats though, it's the secondary ones.
Blizzard cannot balance the secondary stats. A quick visit to Elitist Jerks' classes section reveals that for each class, one secondary stat tends to be far and away the best. Perhaps two are nearly as good or cross paths at a certain point, but there are always those outliers. Maybe its Hit once capped, maybe Expertise just sucks for you because auto-attacks aren't a large amount of your damage. Maybe you're not fond of Crit because your class basically auto crits. For whatever reason, there are those stats that are so much worse than others that any presence means the item gets reforged.
Thus if we had a class who loved haste but hated crit and two hypothetical pieces of armor:
Awesome Chestplate of Awesome
Awesome Chestplate of Pain +1
If your class hates Crit...then the first one is probably the better one. If the second dropped from the new tier of raid content then that content probably isn't going to be as appealing to players. Sure there is the fun of new bosses and new things to do, but can we honestly say we're running every single dungeon because it's just fun to run dungeons? Search deep into your heart...the phat loot is nice. An upgrade is an upgrade and even smaller, incremental improvements will be noticed. It is nice seeing those green numbers and increases on your character sheet.
Now lets go to a fantasy land where the secondary stats are all approximately equal or are so close that reforging would balance out any loss? Suddenly that second piece is indeed an upgrade, and we find ourselves with the answer to our question.
Blizzard has huge item level jumps because they can't balance the secondary stats, not because players wouldn't be interested in smaller power increases.