[snip] a feature called
Wake on Demand. I've linked to the official Apple support document where you can find more info about this feature like how to set it up and how it works exactly.
Good info, and I checked into it. My iMac does support this feature. "Wake for network access" is selected in system (energy) preferences - has been before and after ML upgrade. My wi-fi network is via a 2011 Airport Extreme, and my system profiler (specifically known as "System Information.app") states "Wake on wireless:supported."
However, under ML it just doesn't work- although it did under SL and Lion. The computer goes to sleep after I haven't touched the mouse or keyboard for an hour (as set - the display sleeps after 15 minutes, and I'm no techie but I do know that difference), whether or not it's in the middle of a download or upload. I've been uploading some files for online storage, using Safari, so no worries about "3rd party apps" causing the iMac to misbehave.
Prior to ML, my iMac's sleep settings were just the same as they are now. But my iMac wouldn't stop downloads or uploads in progress at the time sleep was supposed to occur per those settings. And if I left the browser open at bedtime, I would find my fresh emails, notifications, etc., already loaded when I woke up the iMac in the morning. None of that happens now.
I've sent in a bug report on the Apple site. I know, I won't receive a personal reply to that. I don't have time to fuss with any other method of contacting them. I've done both SMC and PRAM resets, checked all settings, read the cited article. It's just not working like they say it's supposed to. I'll try to live with it, but if it's not fixed I suppose I'll have to live with setting "computer sleep" to "never" if I want a download, upload or application task to continue to completion more than an hour after I stand up from my desk. And remember to change the setting back again.
About application tasks - having something like a batch file conversion or DVD file ripping job in progress also doesn't prevent the computer from sleeping under ML although it did under all prior OS's. Apparently if there's been no physical touch of keyboard or mouse for an hour, my iMac just doesn't care what's happening, be it download, upload, network activity, or an application process in progress - it's beddybye time. I'm just getting started using Lightroom, and I expect to have to set the sleep time to "never" unless I want to babysit mass file imports.
Basically, I have to literally hold the iMac's "hand" or set it to never sleep, if I want it to finish lengthy tasks.
I am so so sorry I listened to my expert friends and did this upgrade.