I think Daniele pointed us to some good 'competitive' info -- and it should be absorbed. I looked at pLog before, and it has a pretty nice system. I like the approaches given for 'plugin' or 'block' extension functionality -- that is, some kind of standard way for defining/building, then including/using, a piece of new functionality. AND, be able to then skin/template it to your desires for your site.
I think you use a term that everyone in 'the market' understands. That'd likely be 'plugin' or 'block'. However, maybe there are 'new' terms we need to come up with to differentiate 'types' of modifications -- or adaptation of terms that already exist. ;)
But you also need to consider some things about that 'average person' who is brought up in these conversations. First, if they can get basically into PHP, then a plugin-type system would take care of quickly exposing new pieces/blocks of code to them with a simple line or two or three. Less lines is easier. If the plugins all take certain parameters (in main body or sidebar, maybe a style object or start/end block callbacks, etc.), this stuff becomes pretty simple to extend, very simple to use, and completely skinnable.
Even the above-average people would benefit from a more structured system for 'certain types' of extending-functionality for WP.
However, if we look at an 'average person' that CAN'T deal with PHP, we shift into a new and interesting world -- one where the admin interface would include a simple 'layout' page, where you could pick a basic layout format (2 or 3 columns say), and then have an admin page per column where you can insert/remove, shift up/down, blocks or whatever you want to call them.
I'm not advocating WP become a *nuke. Far from it. I think the nukes are all WAY too complex for the average user who wants to post away. However, what should be learned from the net is that the 'average user' quickly wants to be able to differentiate their site, using simple color changes, texture/skin changes, columns, extensions, all the way to completely hacked PHP. ;)
Just throwing in my 2c. Oh, btw, I just shifted my index to use a start_block/end_block style of 'coding' to make the index cleaner but allow for all the div wrappering to make it easily stylized. I've duped it (in its at-this-exact-moment form) as http://www.chait.net/index.phps if people want to see the raw code. This is just 'step #1' -- where #2 is to move to a 'plugin' style system where the plugins call the wrappering/styling functionality around their content creation..