Most of the replies have been about taxes.
Also look into health insurance differences, division of assets upon split of relationship, will and estate planning implications, such as ability for the living partner to roll over retirement assets tax free upon the death of the other, to have a claim to remain in the home you are living in, even if another relative gets the capital value, etc.
Many of these can be managed with forms, legal advice, wills, etc., but are quite a bit more complex and don't default to what you may expect like they do if you were married.