- too weird - Plan 9 is very difficult to get used to as a user experience. Novel, fascinating, powerful, yes! But also minimal, impenetrable, and very very different from the UI you're used to as a Unix hacker.
- nonexistent marketing
- too hard to get into initially; I wish they had gone to a public source repository model earlier, although in all fairness the project predates a lot of this.
But, it was still an enormous success:
- UTF8 came from Plan 9
- Linux's clone syscall
- /proc
- numerous other minor things
The main thing I wish I saw more that was in Plan 9 was structured regular expressions. I'd love an awk based around them, or an ssam that didn't basically run sam on the file. The second thing I wish I saw more was Plan 9's per-process filesystem namespaces. Chroot and jail are crude hacks compared.