Sorry, Mika, I've been off the grid for a week and haven't been online at all.
Just to be clear, I was saying that, first, since bumping my autosave interval immediately upon experiencing the problem myself (and perhaps having had a re-cache occur during that posting/problem process), I myself have not been able to reproduce the problem. I subsequently turned BLC off after ccolotti's post simply as an added precaution since BLC is not a critical plugin.
The main thing I've noticed, again, is that my PHPIDS plugin has been registering REQUEST/POST.data.wp_autosave.content calls on /wp-admin/admin-ajax.php on every post and post edit continuously since 3.9, which it had never been doing previously.
I first noticed these alerts at the time I myself experienced the interminable saving problem. Since changing my autosave interval to 10 minutes, I myself have experienced no problem, multiple other authors have made multiple posts and have said nothing yet about having any difficulty (although I have yet to pointedly inquire), and PHPIDS keeps clicking off alerts on ajax.php. After having been gone a week, it's up to 99 alerts or so.
This, to me, points to a rewiring of autosave and ajax in the new 3.9 as being implicated in the process, regardless of the actual autosave interval. Because I raised my interval to 10 minutes and thereafter haven't yet reproduced the problem, I'm imagining it may be related to the specific autosave interval itself; the PHPIDS alerts are ultimately only a response to new code previously invisible or considered benign.
Finally, it could very well be my shared server. Every time I inquire of my dreamy host, however, server loads are nominal and everything is working perfectly; my host, therefore, will never be a problem I can actually address.
I'm confident WordPress will get around to fixing this. The autosave thing is not a new problem. It was a nuisance some time back, prompting a flurry of queries on how to raise the interval; I myself raised it back when to 120.
Plugins may indeed aggravate the problem, but they are not generating previously unseen code calls on ajax.php as detailed above: the root of the problem seems to lie there, at least from all the reliable information I have.