WRT the IPX - it's old problem of overhead vs. scalability. Linux always had smaller overhead. So, if you have hardware manufactured in 1994, it's much better choice, I agree. :->
For the spinlocks, see below. In short: they are slow and they don't scale.
As for filesystems - Linux doesn't have anything comparable to ZFS, and this is a huge drawback - snapshots, clones (great for upgrading), fast incremental backups, checksums, not having to size your filesystems by hand and generally fsck with LVM, to mention a few I'm using. But yeah, for handling PC floppies Solaris probably sucked.