I second the notion to stay away from MeeGo for N900. If you want to try it out, get your hands on an Asus/Acer/Toshiba netbook, download the .img file to a usb stick and boot from the live usb. It'll give you a taste what MeeGo will be like on a handheld.

If you do end up enabling extras-devel (tons of great stuff!), check out the "how-to" for flashing your device. I am confident you will not "brick" your device, nor experience major setbacks by downloading from the extras repositories, however, after a few months of playing around, you might end up with a feeling you need a fresh start with your device. Most of the time uninstalling the unused or less used apps works just fine, but occasionally you'll find reflashing the easiest way to clean the table.