1. Does Guild Wars feel more like an MMO or a RPG? I know it's a mixture of both, but like with World Of Warcraft, that's more of an MMO.
I would have to say more like an RPG, for example you don't get that frustrated MMO feeling of having someone kill whatever monster before you when you needed to kill it
2. How big are the PvP areas?
Biggest is AB at 12v12, but there is Random and Team Arenas at 4v4 and Guild vs Guild at 8v8.
3. After you've finished all of the quests, is there anything else to do? Is it like WoW where it's just as fun to hang around in the towns than to go out exploring?
There is a lot to do after finishing the story part. Elite areas like Fissure of Woe, Underworld, and Domain of Anguish, and of course tons of farming spots. People in guilds and towns are *usually* friendly.
4. Does it matter what order you install the different campaign packs?
No, not at all.
5. I loved World Of Warcraft. Will I enjoy Guild Wars?
I think they are very different games. Even in you don't find you like it at first because of the difference, give it a couple of days/weeks, and you'll start to like it.
Hope that helps mate, good luck in future.