Content that is accessible once you hit end game is exactly what I'm talking about, and I personally would argue that it ends up making for a much better game in the long run.
To answer your question about whether it's "progession," if you mean in the sense that you become vastly more powerful than you were every patch, or in the sense that one piece of content leads into the next and then the next in a repeating pattern, then no, it isn't. But I wouldn't say that the power creep is really progression either. Yes, you may get 10 or 20% stronger every few months, but the mobs and bosses are also becoming 10 or 20% stronger along with you. The end result is that every bit of new content is just as challenging as it would have been had they not decided to buff both players and mobs, and all content that came before it becomes easier and over time, a complete joke. New content also may become gated by equipment, where players feel a need to collect newer items to have a shot at holding their weight. None of these make for a very good game experience IMO. On the other hand, if the same content is used in a horizontal progression method, it can still be just as enjoyable and challenging as if it were used in a vertical progression method, with the differences being that players don't feel that they're locked out of the content until they grind to get new equipment, and instead of obsoleting the existing content in the game, it adds to it.
To sum it up, horitonal progression:
-Allows for an expansive game where multiple activities created over months/years are challenging and relevant.
-Doesn't lock players out of content because they haven't upgraded to the latest equipment.
Vertical progression:
-Creates a game where only one or a few activities are relevant.
-May lock out players out of the new content for a time if new equipment is necessary to take down bosses.
Effectively, these are the only differences between the two methods that have a strong impact on how we actually play the game. The only other difference is in how it "feels," but I would much rather have a good game with lots of content than a game that provides an illusion of "progress" and has very little relevant or challenging content.
Lastly, creating good content that people want to play may not be easy, but creating a wealth of good content that players will want to still go back and experience even after new content has been created is something that some would want developers to strive for in an MMO, and many developers aren't even trying. You mentioned you hadn't played Guild Wars, and this was actually one of the best things about that game IMO. The level cap never raised, and equipment never crushed what came before it, and there was lots of content that players could go back and experience from launch day until the latest patch. And not just in a "hey, I'm doing things the retro way like nobody else does any more" like in present day FFXI. Last I played the game two years ago (it's a 2005 game), there were still people playing and doing content from all different time periods in the game's life. That's the type of thing I and some others would like to see developers strive for, a game with lots of content that players want to play that can stand the test of time and not be judged only by its latest patch.