Other way 'round, bosses get stronger the further into a game/xpack you get so I want better gear to be able to beat them so I can get to the next boss who will require even better gear etc etc.
If that difficulty line is flat you lose the feeling of progression and accomplishment which is what MMO's are built around. If there's no progression to follow, what is going to be the carrot on the stick that gets you to keep playing for another month, year, 5 years?
I was talking from a design standpoint. Tier X gives bosses Y amount of median health (or average rDPS / time before enrage or other wipe mechanic), so the gear for the tier must match the rDPS expectations to complete, if that makes sense. From the player point of view your numbers are getting closer and exceeding the expected DPS you need to kill before enrage. etc.
I care about progressing with my character and improving my character. If there is no improving to be done,
The focus in GW2 is on skill rather than gear. The learning curve for some of the content requires a f*ck ton of skill, both twitch/reaction and planning/strategy. The designers certainly feel that progression from advancing skill (and variety of play) will be enough to entertain their audience. We shall see!
Now. Something that Therion and mine's guild from WoW states in their charter (or whatever you want to call it) is that gear is a means to an end.
Oh, if only that were true. The shift to gear-whoring was one of the reasons I left SM aside from an alt. It happens to most players in gear-treadmill games. Not judging, it's just not the mentality of which I want to engage in during my limited entertainment hours =)
Something else worth mentioning (and I'm NOT trying to sell anyone on GW2, more of an interesting trend in modern mmo design philosophy) is downscaling. You effectively cannot vastly over level any of the zones. A bit, but nothing like most mmos. You get the thrill of actual challenge in the combat etc, but also scaled rewards to your character level. This effectively gives you the entire game world to viably explore. No more rofl-stomping The Barrens and get nothing but rep. =) I honestly foresee a game, someday, that will not have character levels as we know them now (the DnD model).
Finally: badgers are best with ranch, just like babies.