There's also two distinctly different solo objectives/methods which often get lost in the discussion. There are distinct differences between being able to blow through large numbers of lt blue mobs at a high rate of speed, and being able to solo mobs at or above your level consistently. There are advantages to both as well (exp gain tends to be a bit faster with the former, but you'll get better drops relatively speaking with the latter). This obviously only becomes an issue at all at higher levels, but as several people have pointed out, all soloing issues only matter at higher levels when mercs are involved.
The classes people tend to talk about as good soloing classes are often those better able to take on higher relative level mobs. Necros and mages specifically are good at this, with other classes having some capability if they use the right tactics. Again though, it's about what you're trying to do. My wizard can kite relatively high level mobs, but the amount of time/mana required makes this a poor exp gain choice and most stuff I'd be even considering loot on will almost certainly summon, so it's not something I do very often. In contrast though, I've found that with GL buffs on (specifically hp regen) my wizard can "tank" lt blues with ridiculous ease and at a silly rate of speed. Exp per mob isn't incredible, but kill rate more than makes up for it (We're talking mobs I can kill with one crit). A guildmate joined me with his necro while I was doing this, and he said he felt more or less useless. By the time his pet could engage anything, it was dead. Dots were pointless, obviously. He basically got in the habit of running ahead and agroing the next mob or two ahead of us knowing that I'd come along and just clean it up a few seconds later. This way he actually got to do something useful.
Point being that there are radically different styles of hunting as well, and even choice of camp at a given level will (and should) vary significantly based on the class you're playing. That necro would never have chosen to hunt where my wizard was hunting, and I'm reasonably sure I'd never pick the spots he solos in either. This means that how well a class solos can also vary based on level, not because of changes in spells/abilities (although there may be some), but because the content changes. In any 5 level range, the spell sets most classes get tend to be pretty constant, with core abilities being retained/upgraded along the way. But the content in any 5 level range will vary quite a bit. Mob makeup in zones vary, and just the theme/style of an expansion can affect things, sometimes in subtle ways.
The most obvious class affected by this is probably the paladin. As mentioned above, if fighting undead of the right relative level and the right mix, paladins can solo very very well. But if the conditions aren't right, they solo pretty poorly. And "conditions not being right" can be as simple as "not enough undead in a close enough geographical area", or "no undead at all in a given level range". For undead hunting to be effective for a paladin, basically all the mobs in a given camp area have to be undead. And there have to be enough undead to occupy the paladin at the relative level he's at. Even if there's a nice camp with nothing but undead in it, if the paladin can clear them all in half the time it takes them to respawn, he's left sitting around either waiting for respawn, or running around slowly fighting the non-undead mobs in the adjacent camps. This gets tricky given that it seems like many times the devs view undead in zones, not as their own set of mobs but purely as a method to make it harder for folks to just invis to where they want to go. So you'll see camps of live mobs, with occasional roaming undead in between them. Not enough to make hunting them worthwhile for a paladin, but just enough to be annoying for everyone else.
While less dramatic, I think every class has choices like that and camp makeups that are preferred. The thing about soloing, is that you are the sole determiner of where you hunt, so a good part of how effective you'll be is you finding the best camp for your hunting style with the class you're playing at the level you are at. And this is absolutely going to be different from class to class. And that doesn't automatically make some "better" than others. More different.
____________________________
King Nobby wrote:
More words please