. . . or is it set up so that I must have at least one account per site to participate? And if the latter, is there an easy way to keep my profile information synced across all the sites that I am a member of?
I would prefer to have just one login, password, personal description, picture, etc. to remember and maintain.
As long as you have associated accounts, you can choose to synchronize your profile with every site using the button offered when editing:
Your network profile will keep track of all your accounts on SE sites, and OpenID will make sure you only need one login for all SE sites. In fact, as long as you're logged in on one SE site, you should automatically log in on the rest of the sites on which you have an associated account. If not, going to the login page should do it.
A common Stack Exchange account is absolutely needed since the addition of the "Hot Network Questions" tab.
I find myself constantly clicking a link from Stack Overflow to an interesting question on a new-to-me Stack Exchange site. When I want to up vote an answer, I instead get an annoying pop-up.
The Stack Exchange account model does not make sense to users and is a UX nightmare. For example, when signing up for meta to write this answer I accidentally created an account with the wrong gmail OAuth account. Twice. Had to delete the accounts until I found my existing account.
At the very least, the sign up flow should prominently offer to use the current OAuth login that already has 7 various Stack Exchange accounts. That has to be the correct choice a good majority of the time.
