The depth of the combat and the 'social experience' being simplified is why people are angry. DA was supposed to be the game catered towards PC gamers after Bioware catered to console gamers with Kotor, Jade Empire and Mass Effect.
Now, back to what makes it so special. In BG2, from a purely combat perspective, mages play COMPLETELY different than fighters. Mages aren't merely archers who throw balls of fire instead of arrows. They can summon elementals who may or may not be friendly towards you... use combo spells to wreak havoc on enemies. For example, entangle some enemies and then spam the room with poison clouds. That's just the simple part. What makes it so good is the DEPTH. Fighters play different than mages, but fighters of different kits play completely different from each other. A barbarian, who can use armor, will play different than a Kensei, who can't use armor but gets attack bonuses. Let's not even get into dual classing.
Now let me give you a direct example of the complexity of combat and why it is good. You see a room full of golems that vastly overpower you. However, there is a treasure chest full of items that you want to get. So how do you get to it? You could take a brute force approach, attempt to cast strength and defense boosting spells and attempt to fight the golems, or you could use smarts and cast invisibility on one of your guys, have the mage summon some creatures... Make the creatures walk in the room and fight with the golems... While those summoned creatures are drawing the attention of the golems, you use the invisible guy to sneak in the room, grab the treasure and run out.
From a social perspective, what was so good about these games? Planescape Torment wasn't made by Bioware, but used the Infinity Engine. Play that game and see if you can tolerate most of the simplicity that are the stories in console rpgs. With BG2, there are tons of subtleties. For example, you can have a party with two females who are fighting over you and you have to make the choice of turning one down. If one of them gets too unhappy, they might leave the party or constantly complain. If you are a good party and you invite an evil character, he may eventually leave because he is tired of your good deeds, or just start attacking you. There are tons of conversation branches to really flesh out your character as well. The choices you make will determine whether you are consistent with the good, neutral, evil and the lawful, neutral, or chaotic alignment you chose when you began the game.
It doesn't really come across in words, but if you haven't, take the time to play BG2 with an open mind and LEARN the game and its complexities. After you do, you'll see exactly what people are complaining about. I'm the furthest thing from a PC elitist, but I'm not going to pretend like a lot of these games aren't being simplified detrimentally to cater them towards consoles.
