Hi,
TL;DR :
Since you force us to play online, at the very least, please, do not reset dungeon progress when the server loses connection!
Warning : story/gameplay spoilers ahead
So this is the third time this happened to me: the server lost connexion during a boss fight in a dungeon, which reset said dungeon.
The first time was on my very first run, while fighting the final story boss (Lilith). It was a tough fight for me since I was still learning the game but after some die and retry I managed to bring her down to maybe 5% health and… the server lost connection. I had to do it all over again.
Second time was in a regular dungeon, not that much time lost there but still had to redo the dungeon entirely.
And finally tonight, I challenged myself to do the 70+ capstone dungeon with my level 58 firewall sorceress. A bold but challenging attempt but, to my surprise, while going very slowly I managed to make it to the boss (Elias, Hatred Remnant). This took me about an hour because every elite mob pack was a tough fight (and let’s be honest, dungeons are a hassle).
Facing Elias was a great challenge, as expected due to the level gap, and after many tries I was finally getting the hang of it, even bringing him down to 20% life. I was getting there and… server lost connection. Dungeon reset. No more tries for me. If it weren’t for the loot and XP, this hour would have been totally lost for me (that last potion I used was)… I just (rage) quit the game for tonight.
Of course, every time this happened I checked my internet connection which was fine.
So basically: I never asked to play online, I’m really a solo player as I’ve been on all Diablo games. I can live with the mandatory online play, like I did with D3, ONLY if it does not hinder my gaming experience.
So far, my Diablo IV online experience has been littered with connection losses (there were other instances in less critical situations) and frequent lag, and it’s spoiling my fun.
Not to mention other players popping out of nowhere, constantly asking to join their group or finishing bosses or events for me - which could be cool IF I wanted them to. We had that choice in D2 and D3, now that choice is gone. But that’s another topic.
In a nutshell : YOU chose to force online play, following this “game as a service” trend. So we, as clients to this service, would expect the service to be, if not flawless, at least not hindering. So have better contingency plans when the servers do go down. Please.
To be honest, even if MY connection were to go down in the middle of a dungeon, I would expect to reconnect right were I left, since dungeons are a bit of a hassle - did I mention that? Ok, I’m done now 
Cheers.