If a login form is not SSL secured, it is unsafe to sign up for you on that. Non-SSL login forms can easily transmitted into plain text. There are many risks with non-SSL login form. Making your login page secure is a big matter to you.
Main risk is that if an attacker can catch your user name and password he can misuse them like transferring money, buying things and reading of your personal emails. Most of users keep same password for many account the hacker takes benefit of this thing. He can access your another accounts. Not securing your login page is the most common mistake during creating of page.
If your site having heavy traffic you can put your login page as a non-SSL page and submission page as SSL page. However, there are problem with that user is a non-technical person he cannot know that his information is being encrypted or not. The pad lock or the green address bar indicates that the site is having SSL certificate but on login page user cannot see it. He might leave the page. An attacker can easily get your user name and password without knowing you because they are traveling through the internet without SSL.
Let us understand how we can create a secure site login form with SSL. Either make your login page separate, which is secured by SSL, or make your home page as your Login page, which is also secured with SSL. It is convenient to make your home page as your login page because people are likely to bookmark the secure homepage than a separate Login page. The page where user enters his personal information must be secured by https. In combination with an EV SSL Certificate that shows green bar clearly is a possible attack man-in-the-middle/phishing virtually zero.
There are some other options if you do not want SSL at all. You can use OpenID, Facebook login ID or Twitter ID to sign up on another site. Such account provides facility to the user securely login to several third party website and it is authentic.
Securing your login page with SSL Certificate appears very small part of your site security. However, if you do not do that customers will not trust you. Making your login page secure is the success of your website.