The biggest thing I've been missing since I started using FastMail is labels. My workflow in GMail used labels pretty heavily, and I've been able to get pretty close using saved searches and folders, but it's not quite the same.
This has happened to me with labels before, too. I'll do a search for all things that have a label and are in my inbox, and then archive them. I'll then go back to my inbox, and see that it missed something with that label. If I then repeat the search, I get zero results, even though it has the label, is in my inbox, and I can go back and find it. It's extremely frustrating.
I do all of my development on a remote box. All of my work is in a screen session, so I can always reattach to it. Since that box is a lot more stable than my Mac is, it doesn't take much effort to restore my workspace after OS X crashes. I also regularly leave builds and tests running while my laptop's asleep, and, because they're remote, they never cause my laptop to overheat and kick on its fan.
Manta doesn't run on a proprietary OS, but runs on SmartOS, a distribution of illumos [1][2]. In fact, almost everything at Joyent is open source, including all of Manta [3]. As for why not anything else, Manta has been around since 2013 [4], before other similar solutions, and allows you to run arbitrary programs on your data. As an example, some of our customers run ffmpeg on videos that they upload, to produce different variants to then store in Manta.