What I mean is, PlayStation All-Stars has plenty more interaction between the characters outside of fighting. It has things like Sweet Tooth challenging Kratos because the latter knocked over his ice cream, Parappa defending a Pipo Monkey from Spike, the Little Sister from BioShock wanting to play with Sackboy, and Drake stealing pages of the Thievius Raccoonius from Sly Cooper, prompting him to give chase. Even in the opening sequence, we have a scene about Little Sister and Sackboy, plus a moment where Sir Daniel rescues Fat Princess from Evil Cole:
Compared to Smash Bros., you only really see things like that in the reveal trailers, while in-game, they don't really talk or interact with each other at all outside of Palutena's Guidance.
Outside of the character interactions, the stages are all themed after one universe "invading" another one, like a battleground from Killzone being bombarded by Pipo Monkeys, or Hades being felled by the plucky Patapon Warriors. I'll admit, seeing the Killzone robots march on Parappa's hometown and get defeated by Chop Chop Master Onion can get distracting or even stale, but it does sell the crossover aspect of the game.
... Basically, what
Wario Wario Wario
said.
I'm sorry if this got too long, I'm just really quick to defend PlayStation All-Stars because that game had a lot of good ideas, dangit. >:T