For most of the game, I didn't think the game was that difficult. I played a rogue archer, shootin dudes with meh bow, Alistair tanking, Barkspawn deepsin, and Wynn healing. The only time I ever turned it to casual was during the two high dragon fights and the boss guy in the ash place.
Then I got to Orzammar. Just coming from slicing through hordes of demon abominations and Indiana Jonesing it up to find the legendary Ashes of Andraste, and I get my ass handed to me by a group of bounty hunters with little to no explanation, curled up in a ball, and turned it to casual for the rest of the game.
I have a feeling that it isn't so much the game itself was hard, but the mages were overpowered. Like REALLY overpowered. To the point where all the game mechanics were thrown out the window and it became a "which mage can throw a fireball first and win". Being in a group with no offensive mages kind of made the game impossible unless for some godly miracle the enemy AI has a brainfart and decides not to throw a fireball.
However, when you have a mage character, or even better a mage character and Morrigan, the game becomes facepalming easy. When you can Cone of Cold an entire enemy group while another mage fireballs the enemy mages, and then procede to churn out OP spell after OP spell, no one in the group even comes close to taking damage.
Around the second playthrough, when I was playing my mage, it was about the time when I solo'd a revenant that took my rogue 5 tries with his entire party to even have a hope to beat that I realised the game was broken. I loved the story, in fact it was the only reason I continued to play the game.
Luckily, after playing the Dragon Age II demo, and seeing that fireball no longers had a knockback effect, and getting my ass handed to me by the ogre, I can safely say it will be a good game.