Lepel and Lurker, how about the way Guild Wars does it?

There are different skills for different situations.
- normal auto attack
- special attack that does more damage
- a special slash that causes bleeding wounds
- a special blow that causes the enemy to get stunned
- a special stab that interrupts the enemies special attack
- a special attack that does less damage but draws attention
- a whirling attack that damages all enemies around you

In Guild Wars you can only take 8 skills into combat. You have to decide which ones you take and which ones you leave. When you use skills they drain your mana/endurance/or whatever depending on the power of the skills. Skills also have cool down timers depending on their power. So you can not use skills by clicking them fast and repeatedly. You have to use them in the right situation to be effective.

e.g. if you see that your enemy prepares for a special attack you would use your special stab to disrupt him. If you don't you would take serious damage.
When two enemies attack you you could use stun on one taking him out of the fight for a few seconds so that you can finish the second opponent...

Some of the skills above only make sense if you have a party. You could get an attacker off your healer with your attack that draws his attention.