Netscape packet storm bugs. Interoperability with BootP servers. Can't mount servers by IP address. DNS lookup timeout is ridiculously long. Assorted DNS resolution problems.
Nobody seems to be able to get routing to work.
Nonstandard PPP implementation causes problems with BSDI and other servers. Modem on COM4 incompatible with S3 video cards. PPP compression won't work on at least some Xyplex terminal server configs. Can't log on to Sun PPP server, or cause Sun PPP server to crash. No way to specify protocol to use for a specific service.