I finished Diablo 3 on normal with Allison. I was playing a Barbarian and she was playing a Wizard.
All I can say is that normal mode is repetative and boring, with little to no challenge. It is like playing World of Warcraft except they took out all the interesting elements and left in the boring grinding.
The game is unreasonably simple. WoW was already pretty mundane because you never really needed more than 7 or 8 of your skills. Diablo 3 decided to limit you to 6 skills, and you probably only need 3-4. In the case of my Barbarian, I could get away with just 3 skills for normal mobs. The entire time you fight things you just left click nonstop and everything dies in front of you.
Bosses were pretty much big trash mobs with more HP. I didn’t find a single boss interesting or unique, and almost all of them involved just standing in place and left clicking them until they died. For example, when we faced Diablo, the final boss in the game, I was grouped with two other wizards. Somehow, both of my companions died and I was left soloing a final boss that was buffed to handle 3 people. I quickly modified my strategy to stand in one place and left click him. Sometimes, I would shift to the left a few inches to avoid a red circle on the floor. Every now and then, he would grab me and take me to under half health. I pop a 15s cooldown shield wall and lifesteal back to full. Having no party members actually made it somewhat easier, as Diablo stayed in place instead of chasing after the other 2 people. After a few minutes, Diablo died and I beat the game, without even using 2 of my long cooldown skills which I was saving in case I came close to dying.
Overall, I don’t find Diablo 3 to be particularly interesting. However, I feel that I might continue playing it solely because my friends are playing it. I also assume that nightmare will be more interesting.