By your definition, stay away from Liberation Maiden and Mighty Switch Force.
EscapeVektor offers a lengthy campaign (about 10-12 hours), and medals, achievements and high score boards will increase that quite a bit. A bargain at $10.
Dillon's Rolling Western is also very lengthy (similar time) but is very difficult and repetitive (which is to be expected, it's a tower defense game)
You missed the Fluidity sale, but if you get used to the gyro controls it's great fun and about 14 hours for $11.
Of the two RPGs I can think of, Denpa Men is very lengthy but only get it if you like Dungeon Crawlers. Haven't played Crimson Shroud but it's supposed to be considerably shorter (8 hours, play it twice though)
Milder recommendations
X-Scape is DSiWare and only 6-10 hours (an extra 2 for 100%) but IMO it's hands down the best game on the eShop at $8. This is the game that needs a 3DS sequel based off of graphics alone.
Pushmo/Crashmo offer lots of content for your money with level editors and QR levels, but I'm not too keen on Pushmo. Crashmo is much better but shorter and pricier.
Mutant Mudds also offers a lengthy campaign for a 2D platformer (8 hours) but I'm not too enamored with it. The art style is suspect (12-bit? What the fuck is 12-bit?), the game offers no real innovation and it's rather slow paced with some lame level design (leaps of faith and bad enemy placement hooray). Also, the DLC levels are some of the worst atrocities in gaming history, the entire thing is made up of leaps of faith and twitch/memorization gameplay.