How about timeless mistakes:

1. No search (this site) or crappy search (reddit)

2. Unreadably small text

3. Ecommerce sites that don't make prices clear ("call a sales representative and give us all your contact information we can then spam you with")

...

More: http://www.useit.com/alertbox/9605.html

he should check with alexa first because for Plurk things seems to go very good:

http://www.alexa.com/data/details/traffic_details/plurk.com