Perks, Item Attributes, Itemization and Item Crafting

Perk Stacking Rules
If it’s a skill perk it does not stack. So perks like “Keen Berserk”, or “Refreshing Mighty Gavel” don’t stack. Only one iteration on your armor will be in effect. Other perks that give a flat boost do stack like “keen” on your armor + “keen” on a ring will stack. Cooldown reduction perks all stack. It seems that perk with a cooldown like “keen speed” that has a 5s cooldown don’t stack.[1].

Refreshing Move
Refreshing move can appear on both a sword and shield but it does not stack[3][4].

Sturdy
Sturdy can appear on both a sword and shield but it does not stack[4].

Carnelian Gem Threat Modifiers and Hated Perk
Threat modifiers are additive. So when you have a carnelian gem and you have hated you get a total of 315% bonus threat. Because tanks require carnealian gems to tank in some instances many weapons that don’t have hated will generate more threat than a hated weapon. This is because perks that add additional damage generates greater threat as that bonus damage is then multiplied by the threat multiplier.

Examples.
A weapon that does 1000 damage with a carnelian gem generates 3000 threat. If you add a hated perk to that weapon it now does 3150 threat (315%).

The same weapon dealing 1000 damage if you have the enchanted perk which deals 9.5% more damage. Your weapon deals 1095 damage and you would generate 3285 threat or (328%) of your base threat without that perk.

It is important to consider the interactions of how damage is calculated before threat multipliers to determine which weapon is ideal for threat for tanks.

Essentially any perk that increases damage output by 1% is a 3% threat boost. Also to note that damage causes things to die faster threat is only threat.

What is the Difference between Block Stability and Block Stamina Damage
Block Stability is the amount of Stamina Damage that you negate when blocking an attack.
Block Stamina Damage is the amount of damage you deal to your target’s Stamina Bar when they block.[2]

Chosing Perks for Weapons
If you are going to craft a timeless shard it is generally better to force a secondary perk than force a weapon specific perk on a weapon. The potential perks that can randomly appear on a weapon is already restricted to perks that apply to that weapon. You won’t get for example, a life staff perk on a great hammer. So most weaponsmiths and arcanists will tell you that your better of forcing a perk like refreshing move ore trenchant recovery for example and then just rolling with whatever weapon perk you might get if you get one. All armor slots can hold a weapon perk so you usually want offensive perks on weapons that can’t be on armor.

Acedotal Evidence Regarding Legendary Crafting
Several OTG crafters have reported higher legendary proc rates when crafting multiple items in a run (queue up five items and craft all five at once) rather than crafting each of the five one at a time. Something to consider if you have materials for multiple crafts and want to try to get the best possible proc rate on legendary crafts.

595/600 Crafting
In order to craft items and be limited to 595-600 range a player must be 200 in that profession, possess the full set of expertise gear for that crafting profession, have 3 major crafting trophies for their profession, have the appropriate food buff AND one town they own a house in must be running the corresponding profession town buff. If any of that is missing the crafter may not be able to craft a 600 item or might have a broader range of levels they can craft.

Sources
[1] Official Forums
[2] Dev Post regarding Block Damage and Stability
[3] Player Bug Report Refreshing Move Stacking
[4] I’ve tested this myself and it doesnt stack (Sonickat)