It doesn't matter much to me, as i generally just play the single player mode.  
 
But i would pay more for the game regardless.  
 
I love the game!

I'd have paid more for the game as it is anyway, but being able to show off my phat lootz to other people would be pretty amazing. Ah well, I can eagerly wait for the MMO instead.

it would definitely be awesome to play with friends.

I would certainly pay more, but I'm glad that they released the game as is simply because it would take many more months to develop a co-op experience. 
In the meantime, I kinda wish they used some of their profits from this single-player release to develop a 15-20$ co-op expansion before immediately pursuing the MMO route.

Seems like some folks over on the Runic forums are tinkering around with a multiplayer "proof of concept."
Link.
Looks like Diablo 3 has competition.

Its worth more than 19.99$ already, the game is amazing. Co-op would definitely be great and I would surely pay even 40$ for it in an instant.

Hell yeah, while playing I keep thinking of how fun it would be in co-op.