I'm not talking about skills, but the design of the perk trees and choices for each one.

Illusion: this perk tree is poorly designed. Lacking creativity since most of the perks just raise the power of your spells. While quiet casting can be useful for a lot of characters who don't use illusion, you have to waste three perks to get it.

Restoration: I like this tree with lots of unique perks like ward absorb, healing stamina and regenerating magicka. Also good is that there are a lot of branches so you don't have to waste a ton of perks if you don't want the whole perk tree.

Conjuration: good design letting the player chose between necromancy and summoning and some unique things for bound weapons.

Destruction: good idea with branches for each element, but a little boring. I think destruction in general suffers from the lack of spell making in this game.

Alteration: I like the fact you can get magic resistance and absorption from a perk tree.

Enchanting: pretty good, although somehow I think it could have been more interesting.

Alchemy: all the power perks are near the bottom, so the design of this perk tree allows players to easily make potions stronger. The least useful perks are near the top. Kind of poor design.

Pickpocket and lockpicking: I have not really perked these skills, so I can't really give an opinion.

Speech: has some interesting perks if you like the skill.

Sneak: good design. The most awesome perk is at the top and pure assassins have their own branch.

Heavy armor: this is a terrible perk tree. A perk for hand to hand combat should have nothing to do with Heavy armor, and the falling perk is kind of useless, but you need both of those to get conditioned.

Light armor: like heavy armor but better because those two useless perks are not there.

Smithing: not bad because it's a small tree and you don't have to waste too many perks to get the armor you want.

Archery: one of the best perk trees. Perks like zooming in, ranger, quick shot and power shot really make you feel like you are mastering the skill.

Block: another good one. The right side really has it's own vibe and quick reflexes and the ultimate perk adds some fun gameplay.

One handed/two-handed: not bad because players can specialize by weapon type. I think two handed is better because the sweep perk is awesome (there's no equivalent for one-handed), and great critical charge is much more powerful and worth spending a perk on than critical charge.


Thoughts?

Never spent a perk on lockpicking, I prefer carry lots of it and if you are a thief, the pickpocket tree has a good design, I didnt like the last perk because, seriously, steal equipped equipment, even clothes. Sorry if thieves also do it in Morrowind and Oblivion, never went too far with thieves in both games.

I don't think Illusion tree is poorly designed. There is some thing i found funny about Quiet casting: "all shouts are also silent". How can a "shout" be silent.

Restoration - i like this tree, it has many useful perks for any kind of character. Also i like to use healing spells instead potions and food.

Conjuration - i don't like followers or summons/thralls. I also don't like to use bound weapons (i like to be able to decide when i use the weapon with soultrap enchantment). So i never spend perks here.

Destruction - Disintegrate is buggy sometimes, all the other perks are fine as they are. Intense flame ("You start running so i can nuke you in the back") and Deep freeze are really good perks for glass cannon mages.

Alteration - This school provides physical defense (flash spells), magic defense (Magic resistance and/or Atronach perks), utility spells (Candlelight/Magelight, waterbreathing, Telekinesis, Detect Life/Dead, Transmute), mana recovery (Equilibrum), crowd control (Paralyze/Mass Paralysis, Ash Shell/Rune). The best magic school in Skyrim.

Enchanting - Not much to say. I just go for extra effect perk and that's it.

Alchemy - For destruction mage, poison perks are important at higher levels, but i only take perks up to Benefactor for other characters.

Pickpocket - Extra pocket is useful for mages, For warriors, with a lot of stamina, is not really needed.

Lockpicking - never spent points here.

Speech - i never spend point in this tree. Money are easy to make and have limited use in Skyrim. You can bribe and persuade without spending skill points here.

Sneak - I only spend points here for Assassin type character. Shadow Warrior is not worth the 6 points you spend just to get this.

Heavy Armor - Exploring in heavy armor, without Conditioning perk is a pain, but 2 skill points are not a big deal. I never take perks on the right side.

Light armor - Much better than heavy armor. All the skill points you spend to reach the last perk are well spent (left side).

Smithing - the number of skill points spent here varies for each character. It's a pretty well organized tree.

Archery - All the perks on this tree are useful for ranger type character. Steady hand can cause some problems in certain circumstances.

Block - I don't like shields, but i spend some points here when i use 2-handed weapons (all bash perks). Quick reflexes can be confusing sometimes.

One handed and 2-handed - Hack&slash/Limbsplitter, Bone breaker/Skullcrusher and Bladesman/Deep wounds are useless, so i never take them and i use different kind of weapons on the same character. I like one-handed for the attack speed (Dual Flurry) and 2-handed for the ability to block and bash.