USELESS CLASSES:
Basically loses to the fighter and barbarian in every way. The only time I'd consider playing a knight is if I wanted to build them as a dodgetank because of the shield evasion bonus. I think that somewhere along the line, they were going to have you gain +1 Protection PER LEVEL instead of just 1 at first level, but that got dropped for some reason; even then I would consider them not ideal. Also, dodgetanks are much less viable in LoG2 than in LoG1, mostly because there are WAY less evasion-enhancing armors in the game. It's still possible though, if you're min-maxing with a lot of dexterity and know where the Crystal Shield is.
Not so much "useless" as "badly optimized". Their enhanced energy pool does not make up for their pathetic health pool; after you get out of the starting area, you'll be able to find a lot of Etherweed hidden underwater, which basically means that your designated caster can basically be drip-fed energy potions. As mentioned before, magic does not scale with willpower, so the +2 willpower is pretty much pointless. Battle Mages, Farmers, and even Alchemists do their job better than they do, mostly because they don't die so fucking fast.
USELESS SKILLS:
The amount of protection granted by this skill is minimal, and the only other benefit is removing the evasion penalties for wearing heavy armor; you can wear the armor and get full benefit from it even with no points in this skill. Unless you're building a pure dodgetank, evasion only has a minor benefit; take this skill only if you have nothing else to spend points on. Dodgetanks should avoid it altogether because they're going to want to wear the Rogue's Set, which requires no armor skill.
3 evasion per level? Fucking seriously? The speed buff at level 3 is kind of nice, but is it really worth 3 skill points?
Doesn't scale with any stat, has a chance of malfunctions until level 5, the ammo is unrecoverable, you don't get any of the good guns until really late… yeah this skill is complete shit. The only reason you'd want this is so that you can use the Hand Canon, and the ammo is so limited for that gun that it's really only useful for stomping bosses, and you've got potions, bombs, and magic items for that. If you INSIST on taking firearms, for the love of god, do NOT have the same person who's using Alchemy use Firearms, even though that's what's implied you're supposed to do. Late game firearms shoot VERY rapidly but do crap for damage, so you need points in Critical and Acuracy to make up for that and you'll be spread far too thin if you try to put points in all of them.
Oh hey, a throwback to Grimrock 1! While it's true that Earth Magic can be cheeseable by doing things like poison-clouding through grates and poison clouding a guy, then immediately locking him in a force field (which I like to call "The Auschwitz Method"), you'll generally do much more damage with fire magic. Poison is also resisted by more enemies than any other element (while fire is resisted by less than any other). You need 1 point in it to get Ice Shards (although Frostbolt doesn't require it, and is generally a superior spell, even though you can't use that until you get 3 water, 1 air magic) and you need 1 point in it to get the special attack of one specific light weapon, but in general, it's just not worth it. You get the only good Earth spell, Poison Bolt, for free anyway if you have 2 concentration and have expored the Hamlet Sewers.
Mostly more boring than useless, but still pales in comparison to other skills. It's not required for any weapon or other item, 20 health is not nearly enough to justify a skill point, and the carrying capacity bonus is really only useful if you're doing some kind of solo-challenge run or something. If you want something to increase your defense, even Armor is superior to this generally.
So yeah, other than the imbalances, fun game. Clever puzzles, fun exploration, rewarding treasure hunting, awesome secrets, interesting flavor and atmosphere… good stuff.
Oh by the way, don't even CONSIDER dual-wielding unless you're playing a mega-min-maxed rogue. The damage that light weapons deal is low enough already, and dual wielding will make it much harder to pierce enemy armor if you don't have big damage already. Crits and backstabs help too.