Great points. To add some more:

* Don't rely on vendor-specific compression and/or encryption which may vary between different tape drives.

* If you do use encryption, try to ensure you have a way to recover from corruption in the middle of the bitstream (see parent's point on two copies, or CTR/GCM mode, or FEC/erasure coding).

* Index the contents (file names, sizes, at least), or use an archive format that puts the index at the start of the tape (note: tar does not).