I've got two things I'd consider far less important than many new features already mentioned, but since it's an open question I'm gonna put them up anyway :)

[h3]More uniform spacing around various BB-Code elements[/h3]

[hider=Examples]
[u]Lists[/u]

[code]My sample text 1.
[list][*]My sample list[/list]
My sample text 2.[/code]

results in

My sample text 1.
[list][*]My sample list[/list]
My sample text 2.


[u]Horizontal lines[/u]

[code]My sample text 1.
[.hr]
My sample text 2.[/code]

results in

My sample text 1.
[hr]
My sample text 2.
[/hider]

The problem is that these elements build blocks that always start end end on a new line, but behave assymmetrically about how they ensure this: At the beginning, they [i]only[/i] insert a new line [i]if[/i] there's not one already present, but at the end they [i]always[/i] add a new line. This results in the manually inserted line-break after 'My sample text 1.' coming out as exactly that, one line break and then the list begins. At the end however you get two line breaks in front of 'My sample text 2': One is the manually added one, the second the automatically added one.

What you effectively get in terms of behavior in front of list/hr/... is this:

[table][row][cell]# line breaks in editor[/cell][cell]# line breaks displayed[/cell][/row][row][cell]0[/cell][cell]1[/cell][/row][row][cell]1[/cell][cell]1[/cell][/row][row][cell]2[/cell][cell]2[/cell][/row][row][cell]3[/cell][cell]3[/cell][/row][row][cell]...[/cell][cell]...[/cell][/row][/table]
What you effectively get in terms of behavior after the list/hr/... however is this (differences marked in red):

[table][row][cell]# line breaks in editor[/cell][cell]# line breaks displayed[/cell][/row][row][cell]0[/cell][cell]1[/cell][/row][row][cell]1[/cell][cell][color=red]2[/color][/cell][/row][row][cell]2[/cell][cell][color=red]3[/color][/cell][/row][row][cell]3[/cell][cell][color=red]4[/color][/cell][/row][row][cell]...[/cell][cell]...[/cell][/row][/table]
I think it would be cool if the behavior after the ul/hr/... would mimick the behavior in front of it, so basically there'd just be the rule 'the first line break in the editor is only for cosmetics in said editor, but entirely redundant for the generated output' for both places. Basically, if the user inserts newlines after the list, the first of them could just be ignored for the output.


[h3]Ignore certain linebreaks within tables[/h3]

Within a [code][table][/code] element, any line breaks you add [i]outside[/i] of a [code][cell][/code] element are remorselessly added to the displayed output in front of the list. This forces anybody to avoid those line breaks even if they'd make the table much more read- and editable in the editor. I think it would be cool of those linebreaks just could be considered as meaningless whitespace merely meant to increase readability in the editor and completely ignored with regard to the displayed output.
 
[h3]Small bugs (possibly to be continued)[/h3]
[list][*]The [.hr] tag is still displayed as a horizontal line instead of just the BB-Code within a [code][code][/code] element.
[*]The list of online users [i]sometimes[/i] adds an unnecessary comma at the end as if there'd be one more user whose name needs to be separated from the others.[/list]