It depends on what class you are going to play and on what type of play you are used to with keyboard+mouse.
You can NOT use the mouse to move (typically done in games by holding down L/R buttons simultaneously).
You can NOT click on the ground to move in action mode (non-tactical mode).
In order to move, you MUST use WSAD.
To turn the camera you use either hotkeys OR mouse in action mode.
To attack you can either (hold) the left mouse button OR (hold) the assigned hotkey.
In recent patches they changed and added some things since game release.
* There is now a run/walk toggle.
* There is now a mouse look camera toggle.
* There is now an auto-attack toggle.
Archer rogues can move while attacking, so if you plan to play this class you might have very few issues. Mages also have it a bit better since they are pretty stationary while casting and staff twirling, particularly if you have your tanks positioned well.
The main area you will notice problems is with melee combat. The movement can seem sluggish because the game waits for your character to stop a weapon swing before moving. So if you are holding the attack button and also trying to move, you are just going to be swinging in mid-air. There are various skills that facilitate movement around the battle and these help tremendously. BUT, it may take you a while to get used to the controls for a melee character.
It took me several dozen hours to get used to the controls when I rolled a (tank) warrior, but I stuck with it because I enjoyed the character. If you think you are going to have problems, I strongly encourage you to find something in the game that will motivate you to keep going so you do get used to it. I also suggest going through all of the controls and arranging things (assigning hotkeys) so that it is the most comfortable for you based on how you play other games of this type.
Other than adding some quality of life changes (perhaps more hotkeys for things), I doubt they will be dramatically overhauling the KB+M control scheme anytime soon.