Clearly, not all classes are created equal. I remember playing AD&D as a kid, and wanting to play a thief. I read through the thief class description where for the first 10 levels or so, my thief skills would be abysmally low. I’d have a less than 50% chance to do the things that thieves are supposed to do – pick locks, pick pockets, that kind of thing. As a result, I never played thieves in D&D, because I didn’t want to be consistently terrible at my character’s wheelhouse.
Melee classes in fantasy games that include spell-casters often suffer from being under-powered, especially at mid and higher levels. While the spell-casters are traveling to other planes, raising people from the dead and incinerating their foes, the fighter is still swinging her sword once or twice per round. The druid shapechanges into a huge cave bear – the fighter swings her sword.
I get that fighters are a good option for players who want a simple character to play. Playing casters, or classes with a lot of special abilities to juggle, is challenging. 4th Edition D&D, designed to help address problems like this by giving all classes special abilities that functioned in similar ways, ended up striking some players as repetitive. Your function in combat mattered more than your class, and the differences were boiled down to fluff and flavor. Previously in 3.X D&D, and then in Pathfinder, they attempted to address this issue with feats and feat chains providing special abilities like Great Cleave and Whirlwind Attack.
Then there is the “dirt farmer” class that some older games include. The System Mastery guys love going off on these kinds of classes that are clearly not fun at all, but included because of misguided ideas of “realism” or “history.” Why be a cleric when you could be a merchant?
Point is, for forty years of character classes in TRPGs, it seems there are almost always some classes that are clearly less fun than others. At high enough level, that class is normally the fighter. So, what to do?
Fix It In Design
4th Edition’s attempt – replace class with role. 5th Edition D&D – give them all magical abilities. Fighters with superiority dice. Look at level capabilities that other classes have and try to give the weaker classes abilities that, if not equivalent, are at least comparable, while in line with their theme.
If your 10th level mage is flying through the air lobbing lightning bolts at her foes while your fighter is still trundling across the ground waving a sword around once or twice a round, that’s a design problem. Not because “balance” is intrinsically valuable for its own sake, but because in a game based around a group of adventurers, all classes should be able to contribute to similar adventures. It’s a problem if some classes become luggage for their more powerful, versatile allies.
Fix It With Equipment
This is especially an option in games like D&D that become dependent on equipment at higher levels. Often, fighter types (who are usually the under-powered ones) have more equipment ‘slots’ than other classes, wearing armor and bearing shields and often able to simply carry more.
Think about the things that other classes can do in your game, and give the fighter, or equivalently limited class, the ability to do similar things. Of course you don’t want to impinge on what makes other classes special, but just make it so the fighter doesn’t have to be accommodated or left behind every time.
Fix It With Hacking
Go into the game you’re playing, lift the hood, and mess around. Talk to anyone playing a class they feel is under-powered or just being left behind by the other characters. Find out what your player sees as a win, and give them a bit more of that. See when the story slows down because she has to be accommodated somehow, and figure out how to keep her going.
Play Better Games
Not all games use classes at all, and not all games with classes are created equal. If you’re frustrated with the incompetence of your AD&D thief, maybe check out an OSR game that emulates AD&D but benefits from 30 years of design insights.
5th Edition D&D is a pretty good start here. I think that 4th Edition also did a good job, but in essence your combat role replaced your class, and then class became a source of color and some customization options, like a sub-class from previous editions.
Obviously, other games get rid of classes altogether, relying on point-buy systems or using other methods to describe characters.
The idea, of course, is for everyone to be having around the same amount of fun, and engagement with the game, each session. There is just a lot of sub-optimal design out there that gets in the way.