Original preface: It's been some time since I heard from someone who wanted me to know that my characterisation was "almost right," "needed correction," or "wrong." I've learned to ignore people who can't see beyond their own tastes when reading fic and those who want to "help me fix" my characterisation; that said, every now and again, I will encounter someone who hasn't learnt these lessons. Because of this and in honour of Meta Friday, please allow me to take a moment to tell you who Snape is.
Snape is straight. Snape is gay. Snape is bi. Snape is asexual. Snape is greasy haired because of potions fumes. Snape is greasy haired because of nicotine build-up from all those fags he smokes. Snape is greasy haired because he just can't be arsed to wash. Snape is tall. Snape is not tall. Snape is not tall but not short, either. Snape is irascible at all times, never allowing another mood to move him. Snape is tender under certain circumstances. Snape is a man of many moods, even if they aren't shown in the books. Snape is not a crier. Snape is a crier. Snape is an occasional crier. Snape is prone to flying off into rages for attention. Snape is prone to flying off into rages because he's ill-socialised. Snape is prone to flying off into rages because he can. (Snape's rages should never be expressed in ALL-CAPS; that is only for Harry. Snape's rages should always be expressed in ALL-CAPS because Harry's are, too.) Snape is sexy. Snape is ugly. Snape is sexy-ugly. Snape is shagged by no one. Snape is shagged by everyone. Snape does the shagging, thank you very much! Snape's name is Snape. Snape's name is Severus. Snape's name is Sev. Snape is a brilliant teacher. Snape is the worst teacher of all time. Snape is a teacher of many things other than Potions. Snape takes it up the arse—but only if he's being pegged by a female character. Snape takes it up the arse—but only if he's being shagged by a male character. Snape takes it up the arse—two cocks at a time! Snape is guided by a strong moral code. Snape is guided by his own obsessive needs. Snape is guided by nothing. Snape is the Snape we see in canon and only that Snape. Snape is the Snape we see in fanon because we don't see enough of him in canon. Snape is Snape, however he's written, and you can write him in your own way, too, no matter that someone might disagree with your characterisation.
Snape is, that is the important thing to remember: Snape is the character we've come to love, however we characterise him, because this is fandom, and writing fanfiction is about, amongst other things, exploring our own ideas about the characters that move us—so stop worrying about what other people say and write Snape however you think best!