1. "Don't touch me." Malfoy spat.
This is wrong unless you have two things happening: Malfoy saying "don't touch me" and Malfoy spitting. If he's spitting out the words, you have to write "Don't touch me," Malfoy spat. Also note that if you used "he" and not "Malfoy", you would not capitalise "he" after the comma. You don't capitalise things that come after commas, unless they're proper nouns.
2. "Stop being such a baby", said Harry.
This is wrong; the comma that indicates that a speaker attribution tag is about to follow must be inside the quotation marks, like so: Stop being such a baby," said Harry.
The only time a comma is appropriate outside quotation marks is when you are listing statements enclosed by quotation marks, e.g. Harry considered a number of retorts, including "I see, now that Daddy's in Azkaban, you've run out of sayings to parrot", "God, you're annoying -- all you can do is whinge", and "shut the fuck up and deal with it", but none quite worked. "Stop being such a baby," he said finally. Even this is a matter of convention; there are people who will argue that a comma always goes inside the quotation marks regardless of narrative context.
3. All of the following snippets are laughable (with a 'meaning vs context' disclaimer on the third one), and you can really only get away with such writing if you're JKR, who either doesn't listen to her editors or whose editors are doing a piss-poor job.
"You have to admit, that was quite funny," laughed Malfoy.
You try laughing that. Just try it. You'll sound like a deranged washing machine on crystal meth. So unless your intention is to make your character sound batshit insane, you will do well not to make them laugh their dialogue.
"I'll see you next year, Dad," smiled Luna.
It's possible to "smile one's approval" or "smile one's gratitude" but that doesn't refer to speech. It refers to expressing something using a smile only. A smile does not refer to a form of speaking. It's a facial expression. Can you frown a line? (Well, you can if you're pir8fancier's WITC-verse Slytherins with their eyebrow code, but that's different.) You can't scowl words, can't frown them, can't squint them or smile them. Body language is body language, facial expressions are facial expressions and speech is speech. English, motherfucker. DO YOU SPEAK IT?
"You cunt," hissed Bellatrix.
This one is tricky, actually -- some dictionaries have been incorporating "to whisper threateningly" into the definition for "hiss", which once used to mean only "to make a sharp sibilant sound". Sibilants are produced by forcing air through a constricted passage -- e.g. s, z, f, soft th. If your sentence lacks such sounds, it really shouldn't be a hiss. It's not precisely wrong, but it's inelegant if your "hissed" statement has no sibilants.
ETA: "some" dictionaries doesn't mean obscure and unimportant dictionaries; the word's used in the sense of "not all" here. Both M-W and Oxford use 'hiss' to mean 'angry/urgent whisper'. Also, 'inelegant' doesn't mean 'wrong' or 'unacceptable'. The primary meaning of 'hiss' is to utter sibilant sounds, and your mileage will vary when trying to describe something as a 'hiss' -- as ariesathena proves in this voice post, even "e" can be made to sound sibilant.
4. "Well," said Harry. "I suppose we'll have to see how it goes, won't we?"
Take out the speaker attribution tag and you've just written the following: "Well. I suppose we'll have to see how it goes, won't we?" Unless it's your intention to have a longer pause after "well", you're splitting a perfectly good sentence into two, of which one is a fragment. Don't do it. The full stop after "Harry" should be a comma: "Well," said Harry, "I suppose we'll have to see how it goes, won't we?"
Another example: "Malfoy was in the Potions laboratory," protested Harry, "I know what I saw!"
Take out the speaker attribution tag and you're left with: Malfoy was in the Potions laboratory, I know what I saw! Do you see the problem? Two full sentences with independent subjects are separated by a comma -- which is not only ugly but some would also say it's grammatically wrong. That comma after "Harry" should be a full stop -- it can't be a semicolon because you cannot precede a line of dialogue with a semicolon immediately after an attribution tag. If there were no attribution tag and that line of dialogue remained unbroken, you can use a semicolon if the statements are closely related in context or you can use a full stop. And thus: "Malfoy was in the Potions laboratory," protested Harry. "I know what I saw!"
5. I know your English teacher told you that variety is the spice of life and you should use as many different verbs for speaking as you can possibly find in your poor, bedraggled thesaurus. I would like to point out that your English teacher's job is not to teach you how to write well. His/her job is to teach you to use language resources (such as a thesaurus) and to learn to appreciate the breadth of the English language. Learning to write happens in classes geared specifically towards writing, not in Grade 10 English.
Using "said" and "asked" is not simplistic, it's smart. They're non-words, they can help you identify speakers without distracting too much from what is actually being said. Using other words might make you feel like your dialogue is snappier, but it'll only distract your reader. The focus of dialogue is what is being said by the characters. Your reader is supposed to be captivated by your dialogue itself, not by the way it's spoken -- if you have to rely 100% on things like "exclaimed", "blustered", "ground out" and "spat" (not to mention those dreadful -ly adverbs) to make your dialogue come alive, then you're pants at dialogue.
It's not wrong to use a "-ly" adverb now and again. It's not wrong to use a specific speech-style verb when you feel it's particularly fitting. In short, it's not wrong to describe how a character is speaking -- but if you have to do it all the bloody time, then your dialogue sucks. The words being spoken by characters should do most of the job in dialogue, not the words describing the speech.
That is all.
Thank you, and please don't drink and drive.