A really negative side effect I'm noticing is latency issues. My ping has been hovering around 170 average towards the evening and even peaked out at 300 for awhile at one point (compared to this morning where it never exceeded 100). This has introduced a significant delay in issuing commands with my characters and if you've ever played any Blizzard games, you know how much delay can really **** up your gameplay in just an instant.
I don't know if this is significantly higher traffic on Blizzrd's servers or on my ISP, but I do know it's not coming from my internal network. Either way, it's a problem that would not exist if there was an offline mode available.
[Edited by Neo7, 5/16/2012 9:38:14 PM]
And it's not like Blizzard isn't well aware of how to handle massive amounts of traffic, given that SC2 and WoW usually don't break on patch days. Well, due to network problems, anyway.
I've honestly just been enjoying Path of Exile as the D3 replacement, personally. At least the developers there set out to make it MMO-esque and don't claim otherwise.