On the 8080, you could "snoop" on the internal actions of the processor. On the 8085, much of this "status" was covered up and replaced by a terser "read/write:data/code" summary. On the Z80, even less information was *easily* accessible (aside from "fetch/notfetch".
OTOH, you could often get a glimpse of the internal *operations* by monitoring pins at "undocumented" times. E.g., IIRC, it was easy to see 16 bit increments (opcodes) "on the pins".
And, the encodings of many opcodes made it easy to snoop on "select" operations with a minimal of external hardware. E.g., you could use things like the RST's as an economical means of syncing/commanding external hardware without elaborate decoding logic.
[And, the fact that you could jam ANY instruction opcode onto the bus during an INTA cycle allowed for some really clever implementations effectively making chip internals accessible to external hardware "for low cost"]The Z80 (and later 180 family parts) tended to be more *practical* in the signals that they presented to the pin drivers. E.g., extending the I/O space to 64K was a significant win (esp because you didn't have to *use* it that way!)
It would have been interesting to see what Zilog *could* have done with the Z80 had they not been so dysfunctional at that time.
Talk about throwing away your market... :-/