What are the critical differences between the various releases of Cave Story (PC, WiiWare, DSiWare, 3DS)? In particular, I already have a copy (of Cave Story+) on Steam from the Humble Indie Bundle 4, but the 3DS release sounds like the most intriguing—is the 3DS remake worth the $30 if I already own a version?
Cave Story 3D (for the 3DS) is a 3D remake of the 2004 original where all the characters and levels are in 3D perspective. There are also a few "easter eggs" for fans of the older games.
The Full Story
There have been numerous new versions and unofficial ports of Cave Story since the original was released in 2004, so assuming you don't want to know the specifics of the TI-83 plus calculator port, I'll just mention the differences of the five official versions.
Cave Story ("The Original", Platform: PC, circa 2004)
The original freeware version, with 100% original awesome.
Cave Story (Platform: WiiWare, circa 2010)
A new and updated version of Cave Story, with revamped graphics, music, and new playmodes.
Cave Story (Platform: DSiWare, circa 2011)
A DS port of the 2004 original, with some of the new features from the WiiWare version thrown in.
Cave Story+ (Platform: Steam, circa 2011)
The awesomeness returns to PCs, with many of the WiiWare features included, plus some other things.
Cave Story 3D (Platform: 3DS, circa 2011)
Cave Story goes 3D, but remains a 2D side scroller... Maybe a picture will help: