I've been asked multiple times (sometimes by the same person) why I play Guild Wars when the "sequel" is out. The short answer is that they have almost nothing in common and I prefer GW over GW2. The long answer is...
GW is a party-based, heavily-instanced game that is more online co-op than MMO. You interact with other players in towns, but as soon as you head out into other areas or do missions, your party (you + other players + any NPCs) is on its own. It has almost none of the things we usually associate with MMOs. There's no crafting, mail, AH, LFG tools, stats, level grind, or gear grind. The level cap is 20 and you will hit it very early. Your success is determined by your party composition and the builds you use. You can't out-level or out-gear most of the game since most of it occurs at level 20. You can get "max" gear at around level 15-20, earlier if you know a few tricks. Basically, you have to play smart and plan. It's a thinking man's (or woman's) game.
The game is also almost all story (big selling point for me personally). It's not quest after quest after quest. A quest or quest chain directs you to an outpost and you do the mission there. Missions are like dungeons in other MMOs. There's a story, with NPCs and cutscenes and such, and you have various goals and bonus objectives. When done, you pick up another quest that eventually leads to a new mission. It's not an extremely long quest chain with optional dungeons, like other games. It's all story all the time, mostly told through cutscenes and dungeons.
GW2 on the other hand is basically your typical MMO but with a GW skin. It has an AH, mail, crafting, level grind, gear grind, and so on. They give you XP for almost everything and your goal is level 80. They lock a lot of things behind level, whereas they didn't in GW. If you could reach somewhere and survive in GW, then go for it. In GW2, story, gear, and skills are level-locked, just to name a few things. There's a story, but it's completely optional and mostly uninteresting. The story in GW was campy with bad voice acting and questionable writing, but it was far more engaging and interesting than GW2's. Plus, Arenanet doesn't care about it. They purposefully butchered it for no logical reason (removed whole sections, rearranged things out of chronological order) and only recently put it back to the way it was without any explanation as to why they did it in the first place.
GW2 is almost never instanced. They want you to be able to see other players and potentially group with them at practically any time. Not a bad idea, but there's not really anything to do. Nothing's tagged, so there's no need to group or even talk to each other. Most fights can be won with just your autoattack so there's no need to group up for difficulty, which is pointless anyway since anyone can help without actually forming a group. There's questing (they won't call it that but that's what it is), zone events (which are just big zergs), and dungeons that nobody does. If you're really into PVP it might be your thing tho, given that they are really pushing the PVP/esport angle. I don't care for PVP so, no, nothing really in GW2 to interest me.
Plus, GW has tons of dwarfs. GW2 has one.