there's technically no difference with Spotpass and DLC, except in the case of Samurai Chronicles and DOA. Those are NOT downloadable content, but unlockable content. They're already in the game cartridge, they're just unlocked thru Spotpass. True DLC involves adding content, features and game fixes through software that's downloaded on top of the game software being updated.
As for which games they should include DLC, I'd like to see first if they plan on implementing DLC for already published games. If they do that (RE:Mercs), then there's real chance for other lackluster games to shine, in the form of DLC. I'd be genuinely excited then.