Caps are relevant.

Low-end:

• heh: “I approve. This is sort of funny.”
• hi: NaL (Not A Laugh).
• hee: You’re trying to show that you’re gleeful. You’re trying, and that’s appreciated.
• ha: Ok, we’re both interesting enough to sustain a conversation interspersed with dry humor.
• :)  works, but could also be a stop-gap “I am not sure what to say” or “I hope you realize this is not really funny for me, but I can still pretend like it is.”
• :) :) :) (or an unspecified number of smilies) You made me feel really happy.
• :-D is a little warmer. No, Skype-users, regardless of what you say, this still means a grin, and not a laugh.

Medium-end: (Huh? That does not even make sense!)

• hihi: Almost NaL. Requires context to be interpreted thus. Possible to misunderstand as you saying hello too effusively. Don’t use, please.
• heehee: Perfectly valid, somewhat-naughty giggle. Bonus if you’re smoking hot and/or cute.
• eh heh heh: On the creepy side. but if we’ve been naked together or if there is a possibility of us getting naked together, this would count as foreplay. Also, if this is a legitimate League of Extraordinary Gentlemen reference, I love you already.
• har har: Sarcastic. Also says you grew up in the eighties, just like me. But it’s been 22 years already.
• snicker: You’re making fun of me. For real, right? This is most disconcerting.
• ho ho ho: So’s your momma.
• :))  Convincingly funny.
• :-D :-D :-D :-D or :D :D :D :D  You’re showing borderline reactions. I don’t know if you find this really funny or sort of funny.

High-end:

• haha: Definitely ironic. Too calculated to sound like you find this humorous.
• HAHA: “Funny, but I do not have time for this.”
• hahahahaha: “Funny. Will probably react with a witticism/link/youtube video of my own within the next few minutes.”
• HAHAHAHAHAA: “Ok, I would find this extremely funny if I was not busy and would probably respond with a witticism/link/youtube video of my own, but I am a little pressed for time and this is the most I can respond with. Also note the typo at the end, which shows you how busy I am.”
• ahahahah/AHAHAHAH: Same as above, but post-modern.
• Hah hah hah: Totally creepy. In my mind, this is a transcription of heavy-breathing-over-phone.
• muhuhaha/muhahaha: We just shared an evil scientist/dead baby/ joke. We’re cool.
• bwahaha: Slightly more sophisticated version of the above, and I will assume you’ve read Giffen/DeMatteis/Maguire’s run on JLA.
• mwahaha: Fuck you and your portmanteau words. Who do you think you are, Lewis Carroll?
• muahaha: This makes me feel like you’re kissing and laughing at the same time. Please stop.
• lol/LOL: Oh ok, you’re lazy. If I like you, I will just mentally replace this with “hahahaha”. If I don’t, I will totally judge you.
• ROFL: I know you’re not really R-ing on the F, but it’s nice mental imagery if you’re smoking hot/cute, in which case I would probably be thinking of R-ing on the F with you too. If you’re not, whatever.
• :))))))))): You’re trying too hard to show that you are amused. That is not a good sign.

Someone approves.