Likes:
Ash vs Iris (as a champion)
Ash vs Cynthia (the best battle of the tournament IMO)
Ash vs Leon and winning at that!
Ash catching Dragonair/Dragonite, Gengar. The team overall was a strong one.
Ash being a competent battler
Seeing several old companions
Traveling around all the old regions
Disappointments:
Instead of a rematch of Ash vs Volkner (who Ash already beat), it would have been so much better to see Ash beat Flint who he once couldn't even give damage to. This would have set us up to be ready/believe that Ash could potentially take down Steven, Cynthia, etc. It also would have been sweet revenge seeing perhaps Flint's Infernape with Blaze vs Pikachu's 10 million volts clash instead of seeing Pikachu fight yet another Electrivire. Instead of fighting Drasna, it could have been Flint. Instead of Volkner it could have been a gym leader Ash never fought before from the Black/White era (although we saw someone else take on gym leaders he didn't battle), or like an old rival he lost to during the pokemon league from Kanto, Johto, or Hoenn.
Ash vs Steven being 3 v 3 instead of 4 v 4 and not having it last two episodes.
Reserves not being incorporated into the battles. They could have had this similar to a pokemon league and had Ash take on old rivals first before facing the likes of Steven, Cynthia, Leon, etc. so that perhaps the reserves could have shined against people Ash once lost to and Ash could have saved this team specifically for the Champion matches. That way he only uses his new team for those final battles, but old powerful mons could take out rivals they once lost to. I.E. Charizard beating an old rival, Greninja beating Alain's Charizard (instead of it being Leon to demolish him lol), maybe all of the strongest 6 that we all love to say being used against Tobias and winning. Could have had 3 battles full of nothing but reserves. Not everyone would be used, perhaps some would be a fall guy, but still.
I still wish they took the time to have Ash visit all the old regions to take on the E4/Champions so each series could get a send off kind of, or since that would take too much time, have Ash fight one big threat from old regions and using those teams. A battle vs Giovanni for Ash's strongest 6 to take down some crazy threat.