Sessions / Cookies not working for user login, registration or shopping cart
This is a very old question, but I stumbled on it while having the same issue.

In my case, the "rewrite" Apache module was turned off. I re-enabled it with:

a2enmod rewrite


I could access /user, but could not login (it would loop back, with no error message). I assumed that since I could access /user, my rewrites and htaccess were OK. I could login with "drush uli", but no form would work. Even weirder: in some pages that had query parameters (GET, ex: foo?test=1), the GET parameters would not be passed to PHP. I was searching all over the place, and the solution was pretty trivial :)