dtgreene: specialize vs not
mqstout: On the flip side, I see systems as you describe as causing a problem where all the characters become flat and same-ish.
The D&D issue you brought up was an issue only in groups where one player goes maximizer. In groups, like mine, where people play for fun and I GM according to the group, it's not as much a problem. And there has to be a *downside* to multi-classing, otherwise, one may as well just get rid rid of classes and go to a [to me, rather comparatively unfun] open-selection, classless system. If there is no downside, everyone just gets the max of everything. (The big upside to multi-classing is versatility and great saving throws.)
Games done as
Grim Dawn is have huge build diversity, which is why I enjoy it so much. I have dozens of characters that all feel and play VERY differently and are each enjoyable in their own way. There's also craft in selecting a path you'd like to go and good creativity in making an unusual "build" work. (And, with GD, it's pretty hard to make a character that can't reasonably make it pretty far through the game, though some are more challenging than others.)
One thing: Characters won't become too similar in these systems unless the player plays for a while, far longer than needed to casually beat the game. While you can make everyone good at everything, there's still the issue of the time it takes to reach that point. (See, for example, Final Fantasy X, where characters don't become similar until what I consider to be the post-game content. (The game is flawed, and it suffers serious balance issues that make the post-game unfun (from what I can tell from videos), but it is still an example of this sort of system.))
Furthermore, in some of the examples, there are equipment limitations that limit what you can have a character do. For example, in SaGa Frontier 2, each character can only equip 2 weapons and 4 non/weapons, and there's still the factor of needing anima from equipment to cast spells (you can give someone all 6 types of anima, but you will likely have to sacrifice defense to do so, and there's the fact that you might not have enough dual-anima tools and quells to equip the entire party that way). In Final Fantasy 5, you can only equip one ability from outside your current job, so while you can learn every ability, you can't have them all available at the same time. (Plus, FF5 has some interesting synergies between abilities that you might not expect.)
Another thing is that there are sometimes innate differences between characters. For example, in SaGa games, while most characters can do most things, some are better than others. In SF2, for example, some characters get more WP regen, only certain characters can use martial arts for some reason, and there's one character who can't use magic at all (which is a major plot point, though not a spoiler; rather it is this fact that drives much of the game's plot). In Secret of Mana, the two characters who can cast spells use different spells, so they are still very different characters even if they have the same spell levels. (Only the girl gets a healing spell, for example, while only the Sprite gets the spell that steals MP from enemies.) (I could also mention that, in SaGa Frontier 1, as well as in the first two SaGa games, dfferent character races differ fundamentally in the very rules for character growth; there are, in fact, 3 or 4 separate growth systems in these games, and they may favor different set-ups and play styles.)
Actually, the D&D issue, particularly the case of spellcaster multi-classing, is more serious than what you describe. If we ignore the "patches" that I mentioned, what we have is a situation where a level 10/10 Cleric/Mage is basically useless compared to a level 20 Cleric or Mage; the character has no decent healing (Heal isn't available until level 11, and every lower level healing spell is way too weak (another problem with D&D in general, though more severe in 2e and earlier)), and attack spells aren't going to work because of how common enemy spell resistance is, and the fact that our multi-class character is 10 points behind (on a 20 point scale) on hitting the enemy; if the single class character can hit half the time, the multi class character can't hit at all! 2e handles this situation batter; the character would get access to 7th level spells rather than only 5th level, and enemy spell resistance doesn't depend on the caster's level, so attack spells aren't completely useless. (The character would lack access to spells such as Wish and (if we gave Clerics their 3e spell lists and higher level spells) would presumably not get the likes of Mass Heal, but that's OK for there to be some sacrifice.)
Basically, the D&D issue means that, if I want to make a character who is devoted to learning all types of magic, the character becomes useless later on.