What small changes would you make to the unpopular classes?

[Jaxon]

The problem with wizards is that they do horrible sustained damage over time and are therefore not desirable in groups.

I would add a new direct damage line of spells that must be cast in melee range, have medium cast times, and have reduced aggro modifiers. Spells having these attributes would make them feasible in groups but difficult to use to kite or solo. These spells would have damage to mana ratios of 7 and above that would raise a wizard's sustained DPS to be comparable with meele and pet classes.