Shattered Kingdoms

I actually found shaman pretty easy to level the two times I've done it, though one of those I stopped around high master and just quit trying to level. It was pretty easy getting to that point, though.

Even a sprite meleer levels faster than casters, I find though. A sprite class with third attack gets four attacks, and with a moderately enchanted weapon you hit comparable damage to a human landing three attacks.

I've recently leveled a barb along side a shaman of almost identical cha. The barb exp'd roughly four times faster than the shaman. This includes the shaman's casting exp gained from refreshing, healing, maledicting, etc.

Both categories have their ups and downs. No one is stopping a warrior from sitting behind a pet, using a ranged weapon (there is an abundance of good material ones in the game) and quaffing/eating buffs to increase their strength or speed. All you need is a couple of charisma modifiers and a pet that can bash. Most melee characters deal decent amounts of damage, so the grinding doesn't take all too long.

Spellcasters on the other hand gain double experience by casting spells, but have to rest frequently to refill their mana pools. Having a bard tag alongside one or simply using the insane amounts of coin one gains through leveling to energise oneself solves that. Also, I'd never use anything but mirror image and color spray while leveling a mage. Cause light, vampiric touch and later on FoD is the way to go for a necromancer.
SoF is absolutely fantastic for priests, and to a certain extent harm/heal too.

Generally, I feel that leveling is quite balanced as it is. Just a matter of being patient, making sure to leech areas properly instead of hopping around to a new one once the XP slows down a bit and taking a few measures of precaution. I've always found MR suits to be quite nice when leveling casters for example, since half the offensive spells bounce off them. They're also ridiculously easy to get.

Again, "double experience" doesn't last long. You only get that for about two ticks, then you've hit your cap and are only getting exp for the damage your spell does, not for the casting of said spell.

That only affects non-aggressive spells. Aggressive spells give you experience based on the damage they deal.

Cyra is correct, the cap for spell xp is only on non-aggressive spells.