If we are not using any of the HTML5 than it doesn’t make sense to specify an HTML5 doctype.

There is no such thing as an HTML5 doctype. It’s just a blank doctype.

An IMG tag certainly isn’t sensible for the header, although an inline styling on a DIV tag is also a bit nasty, so unless dynamically modifies the CSS you are always going to get nasty code for a changeable header image.

Otto is correct that lists can most definitely be block elements, and in fact they are block by default. I see no reason why they need to be an unordered list though and many markup specialists find it a bit hacky to even refer to them as lists at all. The late Dan Schulz who was probably the leading expert on WordPress markup until his death not long ago, often complained about the shonky list markup used in sidebars and comments areas in popular themes. There’s plenty of others who feel differently though. I think the problem there is that there is currently no good option, and so there are a combination of different ways that things could be done.