From what I've seen so far, you really have to put aside those genre-generalizations, because this game really belongs to an fairly new paradigm. Many of it's features are borrowed from several different genres, with a general 'high fantasy' setting. My personal opinion is that, if this works out, you are going to start seeing many more games made in this format versus the old clunky 'massive server farm' format of the old MMORPG.
Basically, you can do either. You can play the game without ever engaging in PvP (except for a brief encounter in the beginning), or you can play completely PvP if you want - and you can do so at any time, without being 'locked in' to any kind of style of play. To truly appreciate how different this game is from the current crop of MMORPGs (which Guild Wars claims NOT to be), you have to examine and understand the rationale behind many of the 'features' of those MMORPGs.
I'll give you an example, because you pointed it out - the level 20 cap. In a traditional MMORPG, you start at level 1, and slowly grind your way up the ladder to get to level 100 (or 50, or 255, or whatever the game is - the theory is the same). Why? So that you can access that new cool area. Or equip that shiny sword you found. Or access that cool new area to kill some monster so you can find that new shiny sword. The underlying theory is the same - repetitive action so you can get over that next hill, so you can see new stuff. And of course, so you will continue playing for months on end, earning a server administrator's paycheck.
When you think about it, however, you realize that it's all just a big timesink. Between those 'interesting' bits that you are working so hard to get to, there's a lot of useless monster killing and go-fetch quests that amount to nothing more than space filler. A LOT of space filler. In fact, probably months worth. In a typical MMORPG, there are many quests you can do, and areas you can see, but they are restricted arbitrarily by what 'level' you are. Basically, what Guild Wars has done away with is the pointless filler space, allowing you to see all those cool things without all that wasted time - basically, you get the fun stuff. You still get the challenge, because everything can be balanced for a single character level - 20 (basically, levels 1-20 are all 'tutorial' missions to get you up to speed). However, you get to do ALL the quests at any time. You can go to all the different areas, after you find them. Exploration actually has meaning, as opposed to a traditional MMORPG where a different area, besides nifty new creature models (sometimes), is just another monster killing ground for higher level players. It's the same amount of content as those games - but with a completely different psychological impact. Rather than pointlessly grinding away for hours, your actions seem to have MEANING because you are doing them for your own reasons.
I'm almost positive that they hired a psychologist or two on the team to create guild wars, or perhaps these things were so obvious that it's just amazing no other company has tried it until now. Regardless, it will be interesting to see Guild Wars - and its successors - and see how well they fare as the next generation of the online interactive roleplaying game.