Is disenchanting an item the only way to learn enchantments?

Solution 1:

Disenchanting an item is the only way to learn the enchantment.

However, the strength of the enchantment isn't important. Once you've learned the enchantment, its strength when you apply it to your own gear is determined solely by your own skill/perks. For example, you can learn the Resist Magic enchantment either by disenchanting a Resist Magic +1% shield or a Resist Magic +40% shield -- It doesn't matter which.

If you've got a sweet piece of gear with a strong enchantment that you don't want to destroy, just keep your eyes open for a weaker/cheaper one. Merchants occasionally have enchanted items in stock, so be sure to check them.

Solution 2:

Disenchanting is the only way to learn enchantments.

Yes, it is annoying - I am currently at level 40 and have seen no resist frost gear.