Seems to me that those who've played Thousand Year Door and the earlier style of Paper Mario games, hate Super Paper Mario and Sticker Star. I'm not surprised as from the looks of things they're two completely different types of games. SPM and SS are being judged by previous entries' criteria, rather than their own qualities.
I've only played the two more recent Paper Marios, and I ADORE both. I loved Sticker Star, every level had it's own charm and personality and it was so much fun making your way through the various worlds. I'd probably hate Thousand Year Door as much as I hate Mario & Luigi Dream Team, the most boring and stale 1st party Nintendo game I think I've ever played.
I don't get how people can dislike Sticker Star, ok the battles aren't amazing (compared to Dream Team at least), but the whole game oozes charm, looks gorgeous and plays superb, IMO.