With each site having it's own rules for passwords, it can be very hard to remember what variation you used on any given site.
I disagree. It's important to pick a strong password to start, and should work on any site. Also a bad idea in general to use the same password and email on multiple sites. (From experience, I Googled my email address once, and found that someone posted the logins for 10k people from one site I visited. I was indeed using the same password on two other sites at the time. Luckily, none were broken into.)
OpenID is great on sites that have adopted it, but it's still not standard.
That's what time is for. More sites will adopt some sort of system similar to this. Especially with the rapidly evolving web technologies, and social media, it will become a standard. Just wait.
What concerns (i.e. spammers, bots, etc.) would make this impractical or unsafe?
You would really need to implement some sort of delay, perhaps one second, between when someone can log in (prevent bruteforcing).
Is it doable?
Anything is doable. You would have to code this yourself, just like any normal login form.
In all, you there are so many other reasons NOT to have email-only authentication.
- Email addresses are public. Couldn't you use your friend's email address on the site and hope he also is a member? That's almost like leaving a box of explosives in the center of your town on a bench.
- What happens if someone changes/cancels their email address? Do they need to register a new account?
- Password hashes and salts are stored for user security (encryption of data by salt?) Using an email address is like leaving the keys in your ignition with the windows down.
Please, don't make the internet any more of a dangerous place.