> To more closely resemble the 2ch look, how about prefixing thread title headers in the main board page with a 【position:postcount】thingie?

2channel does not do this either by default. It can make browsing a bit more convenient (and I suspect dedicated 2channel browsers to insert & read these in some kind of standardized way) but I don't think that's reason enough to impose it on users by default.

> Of course, this could get screwy if you're using reverse order and out-of-order posts in the URL, so I dunno how well it could be implemented.

Personally, I find the reverse order listing, as well as the random order listing, to be a bit silly & useless. The only useful bonus feature here seems to be the comma range seperator, but it seems even in that case there is not much benefit to it (saves 1-3 links in the average case that it is needed, which is rare to begin with).

> The "First 100" link should also be removed from the bottom of individual thread pages, and there should be a link to to thread-list included below the reply box of each previewed thread on the front page.

signed

> In order for the CSS selector not to take over the entire header, how about turning it into a drop-down menu?

This was proposed before (long time ago) and it is hereby also signed.

> and would something like this work (given that all boards share the same root directory)?

That's a tricky bit and I think it was decided against because it would be too much work to properly maintain such a function at the time when 4chan implemented it.