This is the most fun free-to-play gets.
-There is no character customization when you start. Instead, you choose a character. Each has its own class, and each one has an appealing style. There are some you have to purchase in game, or in the cash shop.
-You are offered a decent tutorial, but the game is quite simple to understand with or without it.
-Instead of point and click action, you actually get to move your character with the arrow keys and attack with A and S (plus SPACE for finishing move). I believe this is what makes the game enjoyable.
-Over time you can choose from a wide variety of skills for each character. The special attacks work much like special attacks in any MMO, however they are well animated and keep up the fast pace of the fighting.
-You will hear a lot of the same music over and over, but the music is really well done and fits the atmospheres.
-You could solo all the time, however there is a simple party system if you want to approach bosses (or every level if you want) with allies.
-Instead of a long walk to the town and back to each area, the game is played in levels where, once finished, you can spawn back into town or go to the next. You can replay old levels as much as you want.
-The story line is interesting. Between every quest there are very nice cutscenes with still animation.
-So far, I haven't found the cash shop invasive. I've enjoyed the game without any premium items.
All in all, the game is very well done, and quite impressive for being free. I'd suggest you try it, casual gamer or hardcore.