Cooldown design, rogue design, and aggro system all come off as a shitty attempt to clone WoW in a single-player RPG, and WoW had pretty shitty gameplay in the first place. Then there's the bugs and incorrect tooltips to make matters even more shit. In DAO all the fun abilities go to the Mage class while the Warrior and Rogue classes have no real interesting abilities outside of specializations. It's all minute variations on how to hit shit for damage, and thanks to animation speed issues a number of these are worse than just auto-attacking. DAO is blatantly copying WoW and WoW did combat better. This is how badly BioWare handled it. DAO also lacks immobilize effects and decided to make all immobilizes into stuns with absurd consequences (Small Claw Traps are crazy powerful, as is Cone of Cold). Cooldown design is also a general band-aid for a lack of decent ideas for abilities so you want to create multiple abilities that basically do the same shit and not have players just spam the good one, but hit a variety of different buttons.
On top of that it is marred by significant issues like the dex-to-damage patch (1.02?) making dual-wielding heavy one-handers worthless, Lethality talent much less valuable, and making dex-maximizing builds (which have absurd defense making them impossible to get hit by non-mages - but you can accumulate 100% spell resistance with gear as a Dwarf or using DLC items - or the extremely rare enemy that uses Perfect Striking, ie. Ser Cauthrien) into strong damage dealers. Mages also suffer from One Stat To Rule Them All syndrome where all they need is the magic stat which boosts their spell damage, heals, magic defenses, health potions, mana potions, and so forth. No reason to ever invest stats into anything other than magic. Then you get Wynne, a Mage which has a special unique ability that is completely overpowered and gives her a ginormous boost to spellpower (+70 towards the end).
And on top of that you throw in a shit system where you regen to full outside of every fight, a potion system where you can rotate potion types (small, medium, etc.) to circumvent the 5 second cooldown on your instant heal, and underwhelming encounter design that seems to believe loads of trash mobs without interesting abilities or good tactics are the way to go. Sure, the gameplay gets a bit better if you add combat rules fixpack, improved tooltips, two specializations sten (because Bioware forgot to give him a companion spec or a spec point at 7), and improved tactics (if you use tactics) mods, deliberately avoid using health/mana potions, avoid using DLC items, avoid using Wynne's special ability, avoid pure dex builds, avoid doing pure cunning Captivating Song Bard stunlock, avoid using Mana Clash, avoid using Glyph of Neutralization, avoid using Force Field, avoid using Entropic Death, probably avoid Blood Mage specialization, Cone of Cold, and Confusion too, and throw in Slinks S3 Ravage mod to spice up encounter design, but by default DAO's combat is mind-numbingly boring and even with all that it's still too easily cheesed if you know what you're doing.