Well, really, I think the big difference at the time was just that it was free. OKCupid was a later followup on the same idea, but most dating sites at the time were pay-to-message at best.
You find very different people on PoF than on the pay dating sites. Or even than on OKC, which forces you to expose more of yourself to become visible.
When this originally came up people were saying that, they use it as a reminder to keep using the site. But you can get the same benefit using a login link or something, no need for all the security issues.
The email itself is actually a call to action, it's telling you your new matches. And it's actually weekly. It looked like this before:
Hello yournamehere,
Thank you for signing up on MM/DD/YYYY HH:MM:SS XM.
Remember your password is *****.
http://www.plentyoffish.com is larger than all other free dating sites combined.
View your latest matches here --> http://www.plentyoffish.com/viewmatches.aspx
1. You can now easily check your messages via a cell phone.
2. Latest signups -- > http://www.plentyoffish.com/lastsignup.aspx
3. Users online in your city -- > http://www.plentyoffish.com/lastonlinemycity.aspx
4. If you have found your match you can delete your account in the help menu.
5. Read your new emails here --> http://www.plentyoffish.com/inbox.aspx
6. Who checked out your profile? -->http://www.plentyoffish.com/whoviewedme.aspx
7. Compatibility Test to Measure Chemistry --> http://www.plentyoffish.com/poftest.aspx
http://www.plentyoffish.com
At some point the password line was just replaced with:
http://www.pof.com is larger than all other free dating sites combined.