To be honest, in this order I hate the login issues:
a) When you can't use an email in the username field.
b) When sites don't tell you which one between username and password is wrong (I don't care what security experts say, I would rather receive more spam, than be constantly frustrated by not knowing which one was wrong in every website I browse once in a blue moon).
c) When the username/password has imposed constraints like (must have lower, upper, number, special character, and the blood of a virgin).
d) When the site doesn't hint you about the username/password constraints that were imposed when you registered.
e) When I don't know which one of my N possible passwords I used (this one is my own fault, and this is the only issue I expect to have).
a) When you can't use an email in the username field.
b) When sites don't tell you which one between username and password is wrong (I don't care what security experts say, I would rather receive more spam, than be constantly frustrated by not knowing which one was wrong in every website I browse once in a blue moon).
c) When the username/password has imposed constraints like (must have lower, upper, number, special character, and the blood of a virgin).
d) When the site doesn't hint you about the username/password constraints that were imposed when you registered.
e) When I don't know which one of my N possible passwords I used (this one is my own fault, and this is the only issue I expect to have).