Well, I have one already that I miss, which is Polaroid TimeZero SX-70 film. However, there is a sort of substitute, even if the old manipulations are no longer possible in the same ways.
On gear, I honestly don't consider missing anything. I have a working camera from 1937, so anything I have newer should be no problem to keep working. I might miss Dead Link Removed or EBAY, should either one stop being a source of good used gear.
Films are a tougher issue. I use some Polaroid pack films (peel apart). While there are a couple Fuji Instant choices, it would be tougher if some of the Polaroid peel apart films (like 669 or 690) disappeared. The Fuji FP100C would work okay for proofing, but this does not hold up to manipulation like some Polaroid films.
In large format, either Fuji or Kodak getting rid of Quickload or Readyload systems would really be inconvenient. While I could go to regular film holders, I am really hooked on the ease of use of packet systems.
I only use labs, so I would not miss particular papers. I would miss having a pro lab in the same city I am shooting. Sometimes that happens when I go on location, so I need to send my film back to the lab I usually prefer to use. Again, this is more a convenience issue. If there were better B/W labs available to me locally, I would shoot more B/W.
Mostly I shoot colour transparency. I really like the look and feel of Kodak E100VS, and don't like the Fuji alternatives. I also enjoy Fuji Astia 100F for some subjects, though maybe I could substitute Kodak E100G. My favourite night imaging film is Kodak E200, mainly for the great reciprocity performance, and extreme push capability. I don't know any substitute, perhaps the new Fuji Provia 400X, though strangely the Kodak reps suggested a colour negative film Portra 800 (didn't really like the results).
I miss AGFA APX100, even though I still have a few rolls. There is Rollei Retro 100, but I know that at some point in the far future this will be gone. Every other choice I might use today in B/W I could probably find a good substitute or alternative . . . basically no worries.
Ciao!
Gordon Moat
Dead Link Removed