Imagine you visit 5 pages regularly = 2 minutes registration, 10 minutes waiting for email with password, 1 minute login and cookie maintaining = 13 +* 5 min wasted time of extra work.
Then every few months you find and change websites. you'll never know what one of ten webmasters is doing with your mail address... and it's just a waste of time. registration is great feature, but if it's a must have it's a big banner in the wind "guests are not welcome".