I've never played Smash TV so I can't really comment on that angle, but the previous iteration in the series, Diablo II, is basically a mouse-driven action game where you pick a character class, each of whom has unique skills. Unlike Borderlands where each character has one active skill and a bunch of passive skills, Diablo characters have a combination of active and passive skills that confer different sorts of abilities.
For instance the Diablo II Paladin (the class I played in that game) has an active skill called Zeal that causes each click to perform multiple swings of his primary weapon, each with a bonus to accuracy and damage. He also has access to passive skills called auras that confer various benefits on him, such as Fanaticism that increases his damage and attack speed. Those skills are unique to the Paladin - other classes like the Barbarian and Sorceress have completely different skills and skill tree progressions. So really as far as builds go the game offers a good deal more variety than something like Borderlands.
My understanding of Diablo III is that it uses a somewhat simplified version of Diablo II's skill system - you unlock skills via a tree, but you can only have at most 3 passive skills and 6 active skills available to you at one time (you need to pick which ones, obviously). Both Diablo I & II featured character stats that you could assign manually (Strength, Dexterity, Energy and Vitality) but it sounds like they've done away with that stuff for Diablo III.
I did pour hundreds of hours into Diablo II - I got at least one character into Hell difficulty (the equivalent of Borderlands' third playthrough I guess?) and enjoyed every moment of it. I'm still not too sure what I think about Diablo III since they still seem to be changing a lot of stuff, but I'm also keeping an eye on Torchlight II (www.torchlight2game.com
) which is by some former Blizzard devs.