Evaluating Plan 9 (and Inferno)
There are a lot of things in Plan 9 which are not improvements over conventional Unix... but it seems to me that this is partly because it's stuck in its tiny little niche and never got the rough edges worn down by exposure to millions.
I was a support guy, not a programmer. I started Unixing on SCO Xenix and later dabbled in AIX and Solaris, and they were all painful experiences with so many rough edges that I found them really unpleasant to use.
Linux in the 1990s was, too. Frankly WinNT was a much more pleasant experience.
But while Win2K was pleasant, XP was a bloated mess of themes and mandatory uninstallable junk like Movie Maker. So I switched to Linux and found that, 5-6 years after I first tried Slackware and RHL and other early distros with 0.x or 1.0 kernels, it was much more polished now.
By a few years later, the experience for non-programmers was pretty good. It uses bold and underline and italics and colour and ANSI block characters, right in the terminal, because it assumes you're using a PC, while the BSDs still don't because you might be on a dumb terminal or a VAX or a SPARCstation or something. (!)
Linux just natively supports the cursor keys. It supports up and down and command-line editing, the way Windows does, the way PC folk expect. BSD doesn't do this, or very poorly.
Linux just natively supports plain old DOS/Windows style partitions, whereas BSD did arcane stuff involving "slices" inside its own special primary partitions. (GPT finally banishes this.)