I like the alternative style a lot, it's pretty cool and fun. Mario and Luigi's expressions are great, especially Mario's, he's just completely stumped. I also like the contrast with all the enemies freaking out underground. Their faces are pretty funny, too. That purple turtle guy is really cute. I just wish there was a toilet or something else clear there that they're plumbing.
Overall, I really like the contrast between the mundanity and the crazy stuff going on underground.