The recurring theme: Apple is fighting against cruft — inconsistencies and oddities that have accumulated over the years, which made sense at one point but no longer — like managing to-dos in iCal (because CalDAV was being used to sync them to a server) or notes in Mail (because IMAP was the syncing back-end). The changes and additions in Mountain Lion are in a consistent vein: making things simpler and more obvious, closer to how things should be rather than simply how they always have been.
Reminders in iCal (or BusyCal) are clunky at best, and Notes in Mail via IMAP are absolutely horrible. I’d love Mountain Lion even if the dedicated Notes and Reminders apps were the only new features.
(And if they’d fix the embarrassing reliability of Lion’s Preview.app.)