I haven't played DA:I since December so maybe they patched all of these issues but for me it was:
-Being unable to set detailed companion tactics was a huge step down for one. I hate having to micromanage the party even for relatively easy fights.
-The suddenly limited number of abilities you can use in a fight is another big negative. You're restricted to 8 which not only leaves you unprepared in some situations but is also boring and limits variety and experimentation on the fly. There was no restriction on the previous games abilities, if you knew the ability you could use it. Even on consoles the radial menu could be brought up to use the ones that weren't hotkeyed.
-The tactical camera respects terrain. You couldn't just move your cursor to a hill or to the other side of some rubble, you had to travel your cursor around as if you were a character walking. Pointless annoying and weird.
-My biggest issue however was the pants-on-head retarded ai. For one thing pressing hold position NEVER made the party hold position, certain other commands I issued to my party were also ignored pretty regularly. The ranged characters (even if they had zero short range abilities) would run right up into melee range and get killed. There was one fight I had that was absolutely ridiculous with this. For the dragon fight in Crestwood there's this broken structure with stairs and a few broken walls and it was the perfect place to put the squishy characters (the mages and Varric) so they could be out of the way and I could move them behind the walls when the dragon used a long range attack (it also stopped them from being sucked forward) it worked fine on whichever character I was controlling, but as soon as I switched to a different character, that one would run down the stairs and over to the dragon and proceed to use their long range attacks at a short range. This of course got them killed over and over again until I finally just let them be dead and finished the battle with Cassandra and my mage who I was controlling manually. In every other DA I brought a party made of characters I liked and wanted to hear banter and input from. DA:I was the first game where I brought characters that wouldn't die when in melee range.
-The lack of healing magic and extremely restricted healing items. This didn't make combat more difficult or tactical, it just made things more tedious as well as making certain characters useless. They wanted to avoid potion/heal spamming and instead you have barrier/guard spamming. Your warriors needed to be tanks that built up a ton of guard and Vivienne was the only mage who didn't die from Leeroy Jenkins-ing into a dragon or giant's face. I found rogues to be useless because they didn't have barrier or guard so they died very easily. This was limiting and not fun.
To be clear, I never liked DA combat. The kind of combat I like is the kind that relies on my own skill, aim, reflexes, and use of terrain. IMO though the combat of DA has gotten worse with each installment.