"For the first time, WoW's latest expansion, Cataclysm, only raised the level cap by 5 levels, in addition to the other content it added. Was Blizzard right to make this call? All other things being equal -- the same number of high level zones and dungeons, for instance -- should they have raised the cap by 10 levels? Should the fourth expansion be a 5-level or 10-level jump?"
All prior expansions, all two of them, provided 10 levels. If the only measure of an expansion's worth is in levels, this is clearly only half an expansion. Or maybe less? With the proliferation of heirlooms that speed experience gain, things can take even shorter time. We saw folks ding their 5 levels in a day, including a guildie of mine that earned a Server First for his class. My own casual play only needed a few weeks.
With the new leveling environment and revamped quests in old zones, far more folks will be able to reach the level cap than ever before. I remember tedious grinds of mobs for their XP and that just doesn't happen anymore. Tedious grinding still happens, but not for leveling purposes. Now it's for reputation or gear. This has become welcome to folks that enjoy leveling alts. I no longer feel locked in to my one main and don't feel guilty as I now neglect her while leveling what I hope to be my new main.
A surprising disappointment to me: there isn't a new Hero class! Will there never be another? Is that the lesson taken from Death Knights?
I know there are other current disappointments, especially Tol Barad, but most seem to be something Blizzard is actively fiddling with.
As plenty of others note, levels aren't the only measure of an expansion, and there's plenty of value in Cataclysm. My overall subjective impression is that I'm perfectly happy. I'm very pleased with what I'm getting out of it.