But there has to be at least some power creep with the new specs, that's what sells units, especially in a game where gear is largely irelevant.
I've got to disagree with that pretty strongly. I think people would have bought HoT even if more of the elite specs were weaker than their non-elite counterparts specifically because they might be fun to play. Not everyone wants to be the strongest they can. Some people want to have fun. Once upon a time Guild Wars 2 was only about horizontal progression. Your character never got stronger, but you as a player got better and that was how you grew. I'm positive that regardless of the power level of the elite specs, many Necro players would have bought HoT because Reaper sounds awesome, and many Thieves would have bought HoT because Daredevil sounds fun, and many, many people would have bought HoT because Revenant is a sweet concept for a profession.
They didn't need to be stronger at all. It's 100% one of my least favorite parts of HoT, and GW2 in general nowadays. I personally vastly prefer the playstyle of classic Deadly Arts/Crit Strikes/Trickery Thief and now I have no choice but to play Daredevil if I want to play optimally. In PvP I have no choice but to play Daredevil or I'm letting my team down, even though I'd much prefer the more classic Thief build pre-HoT. I would have still absolutely bought HoT mostly for the new maps, but I would have enjoyed playing Daredevil regardless for a while (even if I would have preferred a Rifle elite spec instead of Staff...).
Content absolutely makes an expansion. You think the people over at /r/wow are happy with Warlords of Draenor because level 100 > level 90? Do you think that's why they bought WoD? 10 more levels? No. I guarantee a vast majority of those people bought WoD because they wanted more areas to explore, more lore to uncover, more skills to play with, more dungeons to run, and more raids to complete, and they're upset because they didn't get as much of that as they wanted, and as a result WoD is very much looked down upon. Because of lack of content.
I don't think, in any way, powercreep is the essence of an expansion.