After a few days thinking about sci-fi and sci-fan I think I have a good way to spilt the two. Here goes.
1. Science fiction is a genre that uses what if scenarios to examine philisophical ideas like the nature of man, technology or even politics. It uses these "what ifs" like a mirror so that we can more closely examine ourselves.
In Frankenstein, Shelly uses the what if of "what if we can bring back the dead?" to create a discussion about who is the true "monster".
2. Science fantasy is really just fantasy but instead of using psuedo medieval europe as a back against which stories are told it uses near or far future settings as a back drop. The focus here is on the characters and the story and although there may be deeper themes in them they are not the focus the way they are in sci-fi.
/snip
Yes, this.
This really doesn't do it for me at all. Whether you mean it or not, it comes across as "science fiction is deep and meaningful, and fantasy is about twinkly fairies." Fantasy can absolutely examine philosophical ideas about the nature of man, etc, and so forth - that's just good literature, not one genre.
I will perhaps posit that sci-fi is more often directed towards examining cultural & societal change, whereas fantasy is concerned with individual & personal change, but that's as far as I'd go.
It's not that fantasy doesn't examine philosophical ideas. You're right, that's not true. But, fantasy most often is examining moral issues, rather than ethical ones. Fantasy is far more concerned with good and evil, rather than right and wrong. That's a whole lot reductionist, but, at the core, I do think that is the basic differentiation.
Time travel was mentioned. I'm currently reading the Black Company series. The book, Bleak Seasons, is a time travel story. The main narrator is being bounced back and forth in time to, essentially, tell three different stories. Time travel is used as a narrative mechanic in order to tell the story in an interesting fashion. It's no longer truly linear. Considering the Black Company stories are written as a "history" book of the experiences of the Company, it changes the way the story is told considerably from the way the earlier books were written.
But, the whole time travel thing is simply a plot device. It has no effect whatsoever on the characters within the story.
Which is the basic difference, IMO, between SF and Fantasy. In SF, the "magic" stuff impacts the narrative. In SF the "magic stuff" drives the narrative. The existence of robots gives us Asimov's Three Laws. The Time Traveler's Wife in indelibly linked to time travel. That the method of time travel is never explained and might as well be magic, doesn't matter. That's not what the story is about. But, it wouldn't work as a fantasy story.
Ender's Game isn't about good versus evil. It's about humanity. Heck the later books are all about what humanity is.