I'm queen of topic jumping w/ natural enhancements from ADHD. Not only do I not care when others do, I'm happy to follow along the winding trail. :D
I play duels nearly as much as I play standard, b/c they're still pretty fun. It's the ongoing deck manipulation & changes that I love so much. I feel like a kid with a present when I get to pick out a new passive.
I HATE arena. I've never been able to get a good deck with the limited card choices. I basically know after the first game that I'm simply not going to win more than once. Duels allow you to build your starter deck, then allows you to change strategy or improve by adding more throughout play. I'm currently able to reliably win 3-5 games, which isn't GREAT but much better than I've EVER done in arena.
The imbalance between available heroes & their powers is still horrible. My current favorites are Vanndar Stormpike neutral or Drek neutral. The 15 spell discovery cards is a life saver for early game decks of Ratttlegore's 5-damage bombs since there's much more padding for the bomb draw chances. Unfortunately it's still very necessary to hit as hard as possible to avoid letting Rattlegore collect way too much armor. The other one that makes me gnash my teeth is the voidfiend priest.
I've only played once in Battlegrounds, and disliked it enough to not play again. It may be more about just being a new format than I'm used to, which would fade after playing a while, but it was an incredibly boring & tedious play mode.
I did like the Mercenaries for a while, but quicky reached the point that ridiculous amounts of grinding must be done before progressing.
I'm a little nervous about the upcoming expansion w/ new class added. The intro of the Demon Hunter class is what led to quitting for so long. It was probably the worst meta that I had personally played. I hope that's not going to recur. DH is still a pain in the ass w/ the ridiculous amounts of VERY cheap hero attacks combined w/ the one of the cheapest hero powers. Coming back to the introduction of DH relics is dredging up the old feelings of utter frustration.