The only reason you can't see noticeable differences between 12 and 28 is that you didn't use about the 90% of 12's features, this discussion is senseless XD
Oh I did, it appears that you're the one that didn't since you couldn't see the down side to it.
RSS, IRC, an AMAZING password manager, customization, an actual mail client (no, the deprecated/dead software called Opera Mail is not), notes, contacts, Presto's RAM consumption, torrent client, the sidebar, dragonfly...
And I might be missing some of them as I didn't used them for almost 2 years, but 28 IS miles away from 12 and that's a fact, so it's pointless to discuss it.
Said that, the last releases seem to be improving a lot the new Opera