If I had to pick a few things to highlight about NetBSD, beyond its portability, I would feature:

1. Xen support. Out of the BSDs it has the best Xen support. I know FreeBSD has some work on making it run in HVM, but NetBSD supports paravirtualization and with 6.0 it will support multiprocessor in DomU. This means that NetBSD will run on Amazon EC2 or Linode without too many workarounds.

2. Puffs. With puffs, NetBSD has access to userspace filesystems and using refuse it can take advantage of FUSE-based filesystems (i.e. ntfs-3g). There is also a bunch of interesting work on moving things like FFS into userspace and having it work as a network service via Rump.

3. Linux compatibility. It can run things like Flash and Skype.

4. Good SMP performance. Although all of the multiprocessor work is not complete yet (i.e. the networking stack still needs to be re-factored), benchmarks show that NetBSD can hold its own against FreeBSD in some SMP benchmarks. In fact, overall, I think NetBSD has pretty good performance while being much more mininal and clean than FreeBSD (this is pretty subjective though).

5. ATF. NetBSD has an automated testing framework for many parts of its codebase and uses it to track regressions. While not necessary a big thing for users, I think it's a pretty cool from a developer's perspective.

Having said all that, I don't currently run NetBSD as my main OS (I did for a few years, about 5-6 years ago), so I could be missing some other cool things. Although I only play around with NetBSD in a VM these days, it has always held a special place in my heart since it was the first *nix I really go into and was the catalyst for me into open source and Unix. I liked that it was minimal and fast and that building the entire userland and kernel was so easy (with build.sh); it's the BSD I tend to identify with the most (hence the logo). Finally, I post a bunch of stuff about NetBSD to this subreddit because no one else does... :)