I'm having trouble with "Set Featured Image" working in WordPress 3.4.2, vanilla install, no plugins activated, Twenty Eleven theme. This has been seen in this post and this post. However, at request of moderator, I'm opening yet another thread on this...
Site can currently be accessed via IP only at 22.214.171.124 until domain name propogation done. Again, this is using Twenty Eleven theme without any plugins active on 3.4.2. In one of the other threads, someone mentioned mod_security as an issue for them - it's not in this case. Tested without mod_security enabled.
admin-ajax.php never returns response to POST for set-post-thumbnail, thus resulting in the issue. Below is the error seen in Chrome console.
POST http://126.96.36.199/wp-admin/admin-ajax.php load-scripts.php:4
f.each.f.(anonymous function) load-scripts.php:4
Network tab shows the outbound POST in pending state before eventually changing to failed due to lack of response. Below is a snippet from that tab.
Request Headersview source
User-Agent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_8_1) AppleWebKit/537.1 (KHTML, like Gecko) Chrome/21.0.1180.89 Safari/537.1
Form Dataview URL encoded