Rosalina's an immortal, or at least has a very long lifespan. In Galaxy 2, during the 120 stars ending, it is revealed that the events of the game are framed in a storybook that she reads to her Lumas. She says to them that the events happened "long, long, ago," and she looks like she hasn't aged at all. She also states at the beginning of the first Galaxy that she visits the Mushroom Kingdom every 100 years. How she gained such a long lifespan is never explained, but it's clear that she's been around for a very long time and isn't aging.
I think the main reasons why a bunch of Rosalina's fans (myself included) come to the conclusion that she is a deity/goddess are her longevity, the fact that she outright states to Mario/Luigi when he meets her in Galaxy that she watches over and protects the cosmos, and that she has shown to have powerful abilities in-game. During the cutscene at the end of Galaxy when Mario/Luigi and Rosalina travel to the center of the universe to rescue Peach and stop Bowser, Rosalina levitates upwards, conjures a forcefield around the Comet Observatory with her wand, and plows through Bowser's airships like it's nothing. And after Bowser is defeated and the universe gets reset, Rosalina appears as a massive figure to Mario/Luigi and explains what just happened completely nonchalantly. She's also seen it happen many times before, because she states, "the cycle never repeats itself in quite the same way". She possesses a very intimate knowledge of the universe, she has a very long lifespan, and she demonstrates powerful abilities. In my opinion, that's why people think she's a higher being. It isn't explained how she gained her abilities, but it is shown in-game that she has a lot of power.
To further hammer in my point, here are a few video links to events in Galaxy and Galaxy 2, so you can see firsthand what I'm talking about.
http://www.youtube.com/watch?v=TosyQEe1qDo#t=185 (Galaxy 1, meeting Rosalina)
http://www.youtube.com/watch?v=CCbdbV7rgWA (Galaxy 1, going to the center of the universe)
http://www.youtube.com/watch?v=mKhXW83AI5I (Galaxy 1, ending after defeating Bowser)
http://www.youtube.com/watch?v=hxZ2pTdxQHY (Galaxy 1, 120 stars ending)
http://www.youtube.com/watch?v=PDBPykUE42M (Galaxy 2, 120 stars ending)