I noticed three big things that seem to affect why I hated the combat system so bad.
1. The movement speed of all the characters seems really OFF versus the attack/spellcasting speed. Sometimes I would be hidden and start off a combat with a wizard spell which in theory would mean a fireball goes off in the middle of the enemy group and THEN they would charge at me. What seems to happen instead is that the act of casting the fireball alerts all the enemies who charge at you like a crazed hyena leaving a big empty space in which your fireball explodes. Similarly my front line guys seem to be unable to keep enemies from strolling past them and running (still hyena like fast) straight for my softest character. The only defense I can see is to have the soft characters just running in circles (also hyena like) to try to play keep away which means they aren't actually contributing to the battle.
2. In BG and BG2 my main character was a gnome thief/illusionist who would hide and scout out the map by himself and locate all the groups of bad guys. Then I would use this knowledge to form up a battle plan of attack from suprise. In PoE I can barely have the bad guys on the same screen as my hiding scout before they are spotted and attacked. I just felt like every battle had to be plowed into rather than allowing a tactical approach.
3. Oh so many battles that have enemies show up once the battle starts who are NOT THERE when you are scouting around previous. So many set piece encounters with enemies that start in a circle around your party preventing any sort of front/back line strategy. I'm thinking of the FRACKING lighthouse (which I never was able to beat) and the cellblock battles in particular which were very hard even without being instantly surrounded.