I just did a fresh installation of Magento 18.104.22.168 and I am seeing some very odd behaviour with the customer login form.
In Chrome (version 36) the login form does not work. I am just redirected to the login page. There is no error message shown on the page. However, when I open a new Incognito Window, I am able to log in just fine. Thinking this was a cookie problem, I cleared out all my cookies from the browser and tried again and I got the same results. In both cases, I am able to log in to the admin panel without any issues.
I have also tried this in both Firefox and Safari. Firefox I would see the same thing happen (but not consistently and I could fix it by clearing the cookies) and I could not replicate in Safari.
I have never had an issue with this in any other version of Magento. Is there some kind of fundamental change in the way Magento handles cookies in version 1.9 and/or is there something I can do to make this more stable?