Are there any special rules for Enchanting?

There are some basic rules that we all know that apply to enchanting, such as:

"An item can't have more than two enchants, and can only have two with the final enchanting perk"

Sometimes in the game, I´ve found myself trying to figure out the rest of the rules that apply to enchanting.

For example, not every item with a magical effect can be disenchanted: there are exceptions, but what are those exceptions?

• Mehrunes Dagger for example, can't be disenchanted for the aprox. 2.5% chance to instakill.
• Masks, can't be disenchanted
• Also, some masks DO HAVE 3 different enchants, also making itself an exception to an enchanting rule.

My question is, is there any place that I can find, or any player that have theorycrafted succesfully about what are those specific rules, defining once for all, what can and can't be achieved/done with enchanting ?

Some unique items have unique enchants (unique enchants are any enchants that aren't listed in yx.'s answer), and these items can't be disenchanted.

Also, unique enchants don't necessarily have to follow the same rules as player placed (normal) enchants.

It's for this reason that certain items can have more than 2 effects on them. And it's also why some items can't be disenchanted.

The reason this is in place is to stop people from creating super overpowered items that have 10 different stats.

It's also there to stop you from finding a good unique item, and disenchanting it so that you can make your Daedric item a better version of that item.

UESP has the entire table for armor enchanting effects.

Copied here:

The two notes are:

1. There are two versions of this effect, allowing it to be applied twice. The second version is from the Shield of Solitude and has a base magnitude of 10%.
2. The magnitude of this part of the effect is fixed regardless of skill, soul used, or perks.

Here are the weapon ones:

You cannot disenchant quest items... Those masks I presume are the dragon priest masks?

Also, some masks DO HAVE 3 different enchants, also making itself an exception to an enchanting rule.

That's not technically accurate. The mask has only a single enchantment, but that enchantment has multiple effects.

You can see this in player-usable enchantments such as Fiery Soul Trap (learnable by disenchanting a unique axe found hanging in some cave or other; I don't recall where I found it), which has two effects: Fire Damage and Soul Trap. Because this is only a single enchantment, you can enchant an item with both effects at any level and, if you have the Enchanting 100 perk, you can enchant an item with both Firey Soul Trap and a second enchantment, giving it a total of three effects. (Note, however, that player-created multiple-effect enchantments only scale the power of the first effect for the player's Enchanting skill and the soul gem used. Any additional effects will have fixed magnitudes.)