Autosave does not equal 'save as draft'....
Time to analyse things...
10% of posts are above average in size (high estimate) and 10% of those posts (high estimate) *may* get lost and need rewritten... as I said, very high estimates and real world will be much much lower... but lets use the above anyway to explain...
Lets say you have 100 posts to make...
10 posts are above average size
1 post (10% of above) could potentially get lost
So, out of 100 posts you could potentially get very annoyed about having to rewrite ONE long post.
Do those figures warrant an autosave feature that will add overhead to the server and slow the experience for EVERYONE? Does it warrant the extra storage requirement on the server? I actually say no, I dont think so. (localstorage is not supported on all devices so a true solution will involve the server)
I think the option to 'save as draft' SHOULD be offered, but not autosave. Admins and staff will use this especially but also many users. If limits were set then this is not only practical but a feature you would expect of any modern forum (I believe IPB is/will be doing this, wordpress and even vBulletin blogs allows this). I feel xenForo needs to lead the field and if not at least keep up.
One other feature I would like to see along side this...
When writing a new thread or reply I would like to see xenForo do an Ajax 'call home' every minute or two... this will trigger notifications. If you are writing a long reply then chances are someone has replied to the thread in that time. If the 'call home' was going on in the background then it could trigger an alert that someone has replied to x thread. This may prompt the user to 'save as draft' and go check the reply before continuing with their response.
Ok, this is just an opinion but I think it would be the best implementation, most useful and leave the smallest server footprint. Autosave would be sucky imo but at least the mod in the post above can autosave on some browsers at client level