Greetings all, prepare for a horribly biased post.
While giving the D3 Open Beta a whirl, I have experienced one of the true problems with making a game like D3 require connections to a remote server; even for solo play. If those servers are busy or inoperable you are unable to play. Likewise if any of the secondary (yet critical) servers or features are busy or inoperable (authentication, database, etc.) you are pretty much screwed.
The gameplay is great, if you can get in. My biggest complaint about D3 is still the lack of an offline play feature.
D3 is less a true product thus and more of a service; think something like GuildWars. A one-time fee paid to gain access to the service, but there is no real product here that you can enjoy apart from Battle.net's servers in any fashion. Blizz may as well have created a Diablo MMO IMO if D3 was going to lack any kind of offline play.
Nuff said. Enjoy it while you can.