My wife and I currently have Prophecies, Nightfall and Eye of the North (as well as the bonus mission pack). Nightfall is the best overall game, IMHO, while Eye of the North reflects the changes you find.
Prophecies has some nice missions to it, and the story is decent...but the overland areas are far apart and sometimes really a pain to navigate. Leveling is more time consuming than it should be, IMHO. Nightfall, by comparison, makes leveling a much faster and more enjoyable affair. Eye of the North is like the advanced course.
What makes GW great are several things, IMHO:
1) Works great solo OR group
2) Not concerned with artificially extending play time via the Grind
3) Separates PvE and PvP totally
#2 is a big one. In any other MMO, travel is meant to suck up a LOT of time. GW only forces you to explore to get somewhere. Once your character has made it there, he can freely travel there by just finding it on the map and selecting it, allowing him to teleport directly there. If your party is in a town, then the leader can teleport the entire group at once.
Here's another one: if you choose HELP, it will provide you, AUTOMATICALLY, a series of links to the GW Wiki
for every mission you currently have. Think about that. Because they've already got your money, effectively, they don't have to worry about making quest info hard to find out. In fact, they do the opposite. You can search the wiki from directly in the game (though you get taken out of the game to see the results). Want to find the collector who takes Silver Bullion Coins? One quick search and BINGO.
And while you cap at 20th, you still get advancement, just not in attribute points. Especially with things like the Title tracks, where you earn bonuses and privileges by certain actions or quests that can provide bonuses in play. Example: in the Eye of the North, the race of giants known as the Norn only respect strength. The only way to earn their respect is by fighting...heck, their merchants and guides won't even help you unless you defeat them in battle, first! Once you've earned a title amongst their people though (by earning 1000+ Norn reputation points and the title Slayer of Imps), you can choose to display their title and gain an addition 50 hit points while journeying through their lands.
The thing I like the most about GW is that it allows me to play the game in my own way at my own pace. We put it down for several months and only picked it back up recently and are having a blast. I highly recommend Nightfall because the quest designs are better, IMHO.
As far as quest length, it varies wildly. A simple fedex quest can literally be over in 5 minutes. A more complicated quest can run from 30 minutes to an hour. Epic quests (usually in dedicated zones) can run over two hours. Nothing in GW, however, is on a time clock (AFAIK) except the occasional instanced mission (like the first mission to reach the Eye of the North, which takes about 5-10 minutes for the first leg) and most can be put-off for as long as you like.
Oh and the occasional holiday events, like the Wintersday festivals, are Hella-fun. I got a Jingle Bear animal companion for my Ranger.
