Well I agree that starting an attack should be easier by simply double clicking on someone for example.
You can do it already by clicking only once by setting custom options for right button
Although making sure thhe server doesn't get lag everytime a large group is engaging would be priority to improve combat in my opinion 
What ought be priority is not the combat itself, but its basic principles. It is eliminating lag effect and
other delays there where something is basically done client side and server have only to check if it is possible. Client should do it instantly and show the effect. In case server would send packet with "no permission", it would be undone. Packets with such permissions should be resend few times in case they wouldn't reach the destination before the effect is undone.
Client could predict some user behaviour and ask the server for information before the time it is actually needed. Like targetting, checking health, names.
While it is not really possible to do this once two fighters are running around and doing completly what they wish... there is huge room for progress for example in the inventory or skill window where everytime you click somewhere, the client is waiting for the permission from server, before it generate the visual effect. Like i said above.
If the computer isn't good and in effect you are experiencing 10 FPS or lower in some areas, don't make the interface behalve the same or worse. Where there is some checks to mark, they should be marked instantly, not with delay, even tiny, so that they won't act slowly maybe even making the sound effect slowed down, giving the sleepy effect. Or you click a button and it remains pressed for a bit like it was waiting for the sound effect to finish playing.
Fixing this, not only combat, mouseclicking in it, but also all other areas of the game will be greatly increased.
