[lazyjones]

> The downside is that it's nearly impossible to go back to a thread and continue where you left off

False.

In the early days, browser designers took care to use different link colors for visited/unvisited links (gasp!). Nowdays, in the age of fancy JS gimmicks and contrast-less design, we are overriding this functionality because it apparently looks better (if you don't see the extra information).

Forums with diffent link colors for visited/unvisited posts and threading work pretty well. It's not to hard to implement something like this with JS (unless one wants all posts to be open/visible as default).