I am glad to hear that you are evaluating mojoPortal. I think you will really enjoy working with it. I will jump right in and answer as many of your questions as I can.
1) Yes, the Site Statistics feature will show you things like Site Membership, Who's Online and even graphs to show membership and online stats over time. Check out the Community page for an example. If you want detailed statistics that include how many times a page is hit or how long a user stays on a page, etc... mojoPortal is pre-configured to use Google Analytics. All you have to do is put your Analytics ID in the Site Settings. See the Google Analytics Integration documentation for more information.
2) You can use Database (Forms) Authentication, LDAP (Active Directory or OpenLDAP), Windows Auth, OpenID, Windows Live Auth, and RPX Instant Open ID. Check out the Authentication Scenarios documentation for more information.
3) FormWizardPro is an addon developed by Joe Audette (mojoPortal Founder) and it will allow you to build forms from within the browser.
4) This is one of the beautiful aspects of mojoPortal. You can create a site, add content to it and then change the design (skin) all you want, your content will not change. Further, your skins aren't tied to any site.
5) If you are referring to upgrading mojoPortal with a new mojoPortal release, this is very simple. All you have to do is backup your current site and database and then copy the new release files to your site. After copying, run the setup wizard and you're finished.
If you ware referring to you making changes to the core code, yes this is possible but you would be creating a Fork of the application and that would make upgrading to new mojoPortal releases difficult. If you want additional functionality, you can request it in the forums, or build it yourself in a custom module (feature).
6) mojoPortal is lightweight in many aspects. It's release files are usually around 15Mb and most (~10Mb) of that is accounted for in the 30+ skins that come with mojoPortal. Once you decide on a skin or create your own, you can delete the extras. mojoPortal is also lightweight in the realm of system resource usage. The built-in minimization of CSS and JS along with the built-in caching really makes mojoPortal scream.
7) Oh yeah...Copy the release files to your server, set your DB settings, run Setup Wizard, FINISHED.
8) mojoPortal targets ASP.NET 3.5 SP1 and all core code is developed in C#.