I rarely feel the need to say "That's not funny" or "I don't find that particularly funny" unless I'm actually offended by the joke. So I think I'd take "That's not funny" to be something a bit stronger than just "not my sense of humour, sorry". Of course, context and tone of voice (and text equivalents of tone-of-voice) matter a lot here.

If I'm confused by the joke, I'll say "I don't get it", or "huh?" or similar. If I just think it's a crap joke, I'll probably just ignore it, or say "okayyy..." and change the subject or tell a better joke. :) Or I might just say "ho ho" in a mildly sarcastic tone of voice. Depends on the person, really.

From the other side of this situation: if somebody said "That's not funny" to one of my jokes, I'd probably be a bit irritated. My response would probably be "Well, sorry I'm sure" or "Well, don't laugh then" or similar. Trying to think about this logically, and think about why I'd be irritated, rather than just imagining what my instinctive response would probably be ... I think "That's not funny" or "I don't find that funny" implies a complaint, which in turn seems (to me) to imply that the joke-teller has some kind of obligation to be funny, that the listener has a right to be satisfied by the standards of humour in the conversation. (Goodness, that sounds pompous, but I hope you can see what I'm getting at.) Whereas I think a joke or witticism is a conversational bonus rather than something about the absence of which the customer^Wlistener needs to complain about. ("The catalogue described this conversation as 'funny', but I don't find it funny in the slightest. I want my money back!")

Does that make any sense?

Yes, perfect sense. Now you mention it, I too wouldn't normally bother complaining unless the joke actually had negative value by being offensive or similar; if it merely wasn't funny I'd just keep quiet and let those who enjoyed it enjoy it.

OTOH, there are situations where you would actually want to bother saying something; for example if you were co-writing a comedy script with someone, in which case - although you don't have an obligation to be funny per se - you at least know both of you want it to be funny or else why are you bothering... The situation that made me write the above entry was another example. Someone on Mono had written something which some people were finding offensive (although I didn't), and we weren't entirely sure whether he was joking or not. I ventured the opinion that if it was a joke it wasn't all that funny, and as soon as I'd typed that I suddenly felt as if I'd accidentally agreed with the people who said it was offensive, whereas in fact I merely meant that it wasn't funny enough to have been worth the rather large amount of effort that had been put into writing it. IYSWIM. So then I had to waffle a lot to make myself clear, and in the course of doing that I got irritated at the English language and had to go and waffle into my diary as well :-)

Someone on Mono had written something which some people were finding offensive (although I didn't), and we weren't entirely sure whether he was joking or not. I ventured the opinion that if it was a joke it wasn't all that funny, and as soon as I'd typed that I suddenly felt as if I'd accidentally agreed with the people who said it was offensive

I know what you mean. I often find myself saying things like "Well, I didn't find it offensive, but it wasn't all that funny," when asked to venture an opinion on that kind of thing. I don't tend to be offended by jokes if they're actually funny -- though sometimes I feel guilty for laughing at them if they're sick/tasteless/etc.

I'm curious to know what the joke was now. :)

I'm afraid the joke (if joke it was - we're still not sure!) was all about the internals of the Monochrome BBS software, so if you haven't used it it wouldn't make sense to you anyway. It barely makes sense to us long-time Mono users, which was one of the reasons I thought it failed to be funny :-)