Here's and example with TinyMCE

Main issue, large amount of blank space preceding a table, and nested tables.

shot 1 TinyMCE html

shot2 result

It's not a show-stopper, we love 1.9.5.5, it's just a bit of a nuisance, as tables are useless now

This is one of the reasons why tables are not a good idea for post content. For Example wrong formatted tables could break the complete forum structure.

The Problem in your case is the Forum Layout uses alot of tables which are styled via the themes.

You could try to override the style, add this to the forum.css or to all theme.css files

.yafnet .postdiv table td{
	padding:0!important;
	margin:0!important;
	}

.yafnet .postdiv table td{
	padding:0!important;
	margin:0!important;
	}

I made the above change; still no joy.

We are a forum of Carver audio enthusiasts, we use tables mostly to share part number, designation and cost tables with each other; occassionally lists of part number to remote codes, etc.

Without tables, the columns don't line up properly

The only trouble we've ever had with tables (previously) was when a member would cut and paste ebay Q&A tables from one of Bob Carver's ebay auctions (those usually have a width variable set, and can mess up the post area).

Something about the manner in which tables are rendered must have changed?

Can you provide a URL to a topic where you are seeing this behavior?

-Jaben