As example sites, for now we provide the TIU site and Jobs Intra Site that are included in Sitefinity installation. If you want to check sites made with Sitefinity by our clients, follow this link
About your questions (answered according to your enumeration):
1. Actually, we use role-based security. Thus, you can give permissions for groups of users. In Sitefinity you can set permissions for pages, for modules, and global permissions.
2. This is possible and quite easy. You can add a login control on a page and in its code programatically. Then, depending on the user's role, he will be redirected to the cms admin or another page (the page set for a specific role). You can set "deny anonymous access" on all public pages and give "view" permission to the appropriate groups of users.
This way, if visitors navigate to a secured page, they are redirected to a login page, then they login and are redirected to either the administration or the other page.
3. This is also possible. However, you'll have to write some code for functionality that is not included in our blogs. If you need a list of all users that have blogs, you'll have to write a simple control that exposes them. To add/edit blog posts not in the administration is also achievable but you'll have to add controls with this functionality (most probably just the same controls used in the administration).
4. I am not sure I understand your request, as I see it, you want to have a list of projects with links to the corresponding sites. This is also easy, you can use the lists module. If these sites are made with Sitefinity you can use the same technique described in 2. for login.
I hope this post is helpful. If you have any other questions, feel free to contact us again.
the Telerik team