Currently, I can only login directly (in any browser) with members of the Sharepoint Farm Administrators group. However, if I login as one of those administrators then login as a regular domain user I am able to login just fine.
Company / Domain: "MyCompany" Regular Domain user login: "UserA" (also a member of AD group "MyCompany\Company Employees") -Yes there is actually a space in the name of the group Domain Admin / Sharepoint Farm Admin / Site Collection Login: "DomainAdminA"
I CAN login as "MyCompany\DomainAdminA" and THEN "Sign in as different user" with "MyCompany\UserA"
I CAN NOT login directly as "MyCompany\UserA".
One thing to note, this is a new installation. I have upgraded 5 site collections from 2007 to 2010 and imported them into the same Web Application - and when I log into those site collections (ie. https://intranet.company.com/sites/collection1" I AM able to login directly. But when I try to log in directly to https://intranet.company.com I am NOT able to login as UserA - only as DomainAdminA then Sign as Different User with UserA.