Maybe I just wasn't very good, but I found Guild Wars (especially the original release) to be not at all solo friendly. You had to have specialized builds to farm certain areas solo, you definitely couldn't run the entire game with any one class/build like you can in Diablo. Also, the game is designed with certain amounts of latency in mind, Guild Wars plays like your typical point, click, wait until the monster dies MMO (even though it wasn't a true MMO). There's no stutter stepping or kiting going on in that game. There would be no noticeable advantage to playing Guild Wars offline, and frankly I don't think anyone could finish the entire game alone, whereas playing Diablo without any lag is a significant improvement because it's a fast paced action game.
The difference between the two is one game was designed from the ground up as an online only title, and it didn't have two previous releases that were online enabled rather than online required. Guild Wars gets easier as you party up, Diablo has always had the difficulty scale up when more players join because it's single player friendly. Guild Wars requires the heavier social interaction side that most MMO's need to survive and draw in their larger playerbases, because frankly the gameplay itself just isn't very good compared to more action centric games like Diablo. Online play in Diablo facilitates the looting and gearing aspect of the game, online play in Guild Wars and most other MMO's is more about community building, hence the names "Guild" Wars and "World" of Warcraft.
If anyone here claims to have beaten the entire campaign of Guild Wars with a single character using only henchmen/heroes I'd like to see proof.