To be clear, CSSU doesn't replace Maemo5, but rather is way to change your update repository. Meaning you would be getting updates from the CSSU team from now on, vs Nokia. If you haven't installed CSSU you're getting updates from Nokia, which are rare and likely to not happen again short of another major certificate issue or the like. Once you install CSSU, your primary updates would come from them. But the core is still Maemo5.

The rest of the list is rather well put together. It's also worth noting that updating to CSSU gets you several of these already (bug fixed modest, camera-ui 2, latest stable kernel, etc.)