After running 4 characters to level 30+ (one level 62), I've found the most essential skill is Smithing. The only way you can avoid it is if you play a pure mage and max out both Enchanting and Alteration.
The least essential skill has to be Speech, followed by Lockpicking. Either are a total waste of perks. By the time you can make use of Speech's merchant perks, you have so much money it doesn't matter if you get 2K for an item or 4K. Lockpicking is great on its own, but the perks are next to worthless. Having the Master Locks perk might be ok, but you have spend a bunch of worthless perks along the tree to reach it.
Why? I've never we used it once. Now I've only played two toons- a archer assassin and a tank 2h barbI never do the enchanting or alchemy in these games. The game is already a time sink and to be honest by level 20 I begin to feel over powered as it is.
What am I missing?
Enchanting is pretty powerful. Just started that game last week. Level 25ish Archer. Between enchanting all the armor and the bow, each shot is close to a kill shot. There is an archery enchantment that can be applied to almost all of the armor which give you +20% damage.ETA: I've found Alchemy pretty useless. No need for potions other than healing (and food works just as well for that).
First off, I'm assuming you're playing on Master difficulty. Smithing is essential because without it your armor and weapons are basically half-powered without Smithing improvements.
Enchanting is important because the top perk allows you to place two enchantments on one item. You can actually put a single +40% weapon damage improvement on four items (helmet, gloves, ring, and necklace).
When my level 62 guy uses Archery, he wields a Dragonbone bow with "fiery soultrap" - +10 fire damage and soultrap, and "fire" +30 fire damage, with +160% Archery damage from his equipment and 100 Archery with all 5 damage perks.
Despite that, it take at least 5 or more shots to kill an Reverent Dragon. He still gets killed now and then.
The best tactic he uses isn't the bow at all, but dual wielding swords with double enchantments each, +160% One-Handed damage from equipment, +100% from all 5 One-Handed perks, and +50% for dual wielding power attacks (it's a perk). I summon a Draemora Lord to distract the dragon, then run around behind him and power attack it with both sword. The amount of damage is staggering. I can kill a Reverent Dragon in 2 hits.