Yes, upgrades are the most important, but I find that some of the nicest "perks" are being treated like a "human" by the CSRs, FAs, RCC people.

The little things like smiles, having people thank you for flying UAL, etc, are some of the greatest. Also, knowing that if my flight is cancelled, that the CSR in the RCC will wait list me on a couple of different flights to make sure I get on my way!