I agree with The Cursed Erkling, Lilly did love Snape and he broke her heart the day that he called her a 'Mudblood'. Why else would she put up with his Dark side? His own heartache is caused by his regret of that moment, and of him handing Voldemort the information about the prophecy.

Had Snape found a loving relationship he most likely wouldn't have become a Death Eater. I think that for Snape Voldemort has that Dark glory, a chance to explore magic in a way that he never could have at school and the chance to have a subsitute family in which he could play an important and powerful role. If Snape had Lilly, he wouldn't need Voldemort.

The bullying and cruelty come from a combination of being the victim of his father and school bullys (including James and Sirius) and the bitterness of losing Lilly. If he had managed to come to terms with losing her he would have been a very different person. She seems to be the only positive person in Snape's young life.

Snape as a father... hmm that's tricky. Most likely he would be strict, instil a sense of self discipline and a sense of ethics on how to treat others.

I tend to think that Snape has been cursed with the hair type that always seems greasy, no matter how recently washed. The fact that he is an expert poitioneer and should be able to sort it quiet quickly and simply shows that he is unconcerned with his appearance, either because he thinks it is unimportant or (as others have suggested) he uses it as a way to distance himself from other people.

When it comes to singing and dancing, I just can't see it happening. Possibly tapping his foot in time with the music. Maybe.