1. Nonsense. Having the code does not make it possible to steal your data as long as you install it and configure security according to best practices. Open source means lots of eyes are looking at the code and if they see any mistakes that could be vulnerabilities they report them. Obscurity does not make security.
2. mojoPortal is not software for social networking sites it is for business sites, see the section at the bottom of the About page which describes what mojoPortal is not designed for. If you want features like that you could build them yourself.
3. You can add custom fields to the registration page and user profile, see the documentation.
4. Create a css file for your custom feature, put it in the skin folder and add it to the list of css files in the style.config file in your skin folder.
5. Nope there isn't, maybe you can build that if you need it but it is not in the box.
6. Each CMS platform has their own requirements for skins, some CMS platforms are more popular and therefore lots of 3rd party skins. We are not there yet, but it is very easy to make skins using Artisteer. Usually for any important site you want a custom skin not a design that other sites may also be using. Any worthwhile site should have its own unique design.
7. Not true, if you're doing that you're doing it wrong. Just upload the new files on top of the old files and then visit the /Setup/Default.aspx page. It will only overwrite mojoPortal system files, it will not touch any custom images or other files you have uploaded. You should not fork the mojoPortal code or modify existing mojoPortal files that way there is no risk in upgrading.
8. Yes, your site looks kind of boring because you are using an old skin that shipped in older versions of mojoPortal and it looks old school and out dated. You really need to make a fresh design to make your site look good. A reminder service is an interesting idea but its not easy to compete with other things that already exist such as google calendar and others, if you search google for "reminder service" you can see there is already a lot of them.
Hope that helps,