I don't like bbcode. The question of blocks is a good one. The markdown way of doing PRE would be a bit impractical in a textarea where the user will most likely not be able to tab large groups of lines, or tab at all for that matter. The Remarkable way seems more user-friendly, but still kind of akward. I've been thinking maybe use something even simpler, something like the Python comment syntax e.g.

"""
code here
"""

Obviously, might make it hard to post Python code if there are block comments in it though. Maybe it's more intuitive for that style to be for block quotes though.

Maybe:

###
coffeescript style?
###

...for code and the triple double quotes for blockquotes?

Yeah, really there doesn't *need* to be a difference between the CODE and QUOTE markers. Either way, they'll both be placed in a rectangular area... the only difference is usually the font formatting. Font formatting shouldn't be that big a deal -- either way, you get the point across with the text box.

The problem with using "«" and "»" is that they are rather difficult to type on PC keyboards (at least I don't know any shortcuts for them). The tilde character is better, but still a bit of a hassle (especially for non-US layouts).

I like the idea about Python docstring syntax as it is simple and fast to type.
As mentioned though, it doesn't nest, and sometimes you'd want to include nested quotes in your text.
How about a combination?
"""
Quote goes here
;;;

%%%
Code goes here
;;;

The exact characters could be decided later, but you could at least use the same end tag for all blocks to make it easier to remember.

Inline quotes could just use "quote here" for instance, and inline code could use the LaTeX Math syntax of $code here$

Hmm, are nested blockquotes and PREs really necessary? Speaking as a user, I would be comfortable with a lack of nested quotes/pre's as long as it provides simple/convenient syntax.

Regarding nesting:
It might be of simply not having noticed so far, but I haven't seen so far any case of code needing to be nested. Code tags inside code tags might happen, but they don't seem useful for me (furthermore, I'd actually disable any conventions inside code tags, with perhaps only linkification being kept, but without converting the text inside as it happen outside).

Quotes are completely different from code blocks. Quoting from these forums themselves could mean having other quotes inside quotes, and these quotes might be necessary for context. In this case, I think it is necessary to have nestable quotes.

So, my idea for syntax is that it should be intuitive. Inline quotes could use "quote here", being able to nest only up to 1 level of quotes by doing "quote 'quote of quote'" (having more than 1 level of inline quoting would be messy). It seems to me that this kind of syntax is intuitive and straightforward.

Block level quotes are more doubtful. Having a non-English keyboard, I have
«Quote»
accessible and usable (although it is not common to use it where I live). But if that is not understood, perhaps
<Quote>
or
<<Quote>>
could be used.

Code is more difficult because there is no single thing that says "I am code!" What I see working perhaps, would be a certain number of cardinal signs. Like so:
###
Code
###
This is because cardinal signs seem to me to have always been related to more technical topics.

Hmmm, I'm still leaning toward simplicity for my vote. Maybe just don't worry about nested quotes/pres. It will even make the regexes faster and simpler. I can't imagine anyone complaining harshly about not being able to nest quotes.

However, if we really wanted to be intuitive about this, it could be possible to have the start of a blockquote be a line feed and a double quote, and the end be a double quote and a line feed:

"Start parent quote

"Nested quote here"

End of parent quote"

That should allow for nested blockquotes, and be fairly intuitive as well. It also for the most part wouldn't interfere with "inline quotes like this one".

Yeah, I realize were not doing a show of hands here. That was just my conceptual vote.

My conceptual vote goes to JJ's suggestion as well ;)
In fact, this could be extended to work for code as well...
Only since it doesn't have to nest, we can use the start-end on it's own line (looks better)
$
#!/bin/bash
echo Hello world
exit 0;
$

"""
i can has cheezburger?
"""
No.

The whole design of the bbcode eventually used must be such that it doesn’t cause people to nag for a preview function. I want to avoid that. It’s one of the very things so wrong with current forums.

That is incredible! :D

Question, does it allow nested block quotes? Who am I kidding? I'm testing it myself! :D

Test:
'The whole design of the bbcode eventually used must be such that it doesn’t cause people to nag for a preview function. I want to avoid that. It’s one of the very things so wrong with current forums.'

That is incredible! :D

Let's try this again...

How many therajes does it take to screw in a lightbulb?

0.35

This is incredibly awesome, and just to test it one more time, I'm quoting here Kroc's tweet with the incredible regexp he used:

Just for reference, here's the corrected code: /(?:^\n|\A)(?:(")|(“)|(«))((?>.|"(?4)"|“(?4)”|«(?4)»)*?)(?(1)"|(?(2)”|»))(?:\n$|\Z)/msiu

Ahh, now I miss real regexes. Damn javascript.