Annoys me when i can't create an account on a site without a twitter or facebook account. Not everybody uses these services.
Then sites started requiring emails to sign up - and you signed up anyway. Why wouldn't sites just let you register through snail mail? They're limiting their user base!
In all seriousness, Twitter and Facebook are trending, and you might as well adopt them. Not doing so would strike me as a bit of technological ignorance.
Note: I have accounts on these sites and actively use them, just my guess as to other peoples' rationale behind not getting an account.
Implement own login system: X hours
Selling your users privacy down the river: Priceless