I am not a huge Mario fan. I don't actively dislike them, but I don't play them much. I do really really like Mario 64 though, which was why I gave Sunshine a whirl.
I think one bit that irritated me was that it was less "free-form." In Mario 64, you gradually unlock areas and ultimately need 70/120 stars to beat the game, but you can get them from all over. In Sunshine, you simply have to get the first seven shines in each of the 8 stages. That kinda bothered me, because I couldn't move on from an irritating shine to try something else; I HAD to get it. On the flipside, there was no point to exploring Delfino Plaza for Shines besides pure completionism. If you're a guy who really loves Mario and always goes for 100%, I guess this wouldn't matter, but I found it irritating.
Besides that, I think the game was frustrating a little more often than Mario 64, and the controls were more frequently irritating. This is also 100% subjective, but I didn't care much for the music, the beach town setting, or the overall "vibe" of the game, compared to other Mario titles.