Some good calls on this. I've a few comments, then my own nomination...
Disk - well yes, it was dreadful, but I really liked it precisely for that reason. I had a Halina camera with a simple meniscus lens that I picked up in a bargain box for 50 pence - essentially almost free. This was so terrible, allied to the tiny neg size and the quality of films at the time, so terrible that results didn't really look like photos - more like impressionist paintings. And I absolutely adored that. It was a very sad day for me when films and processing stopped being available. Very early sub-VGA digital cameras, and especially non-smart camera phones are the only thing that have got close since.
APS - in many ways, its legacy lives on, particularly in the form of APS-C sensors. But timing in particular was bad for APS - yes, digital was just around the corner when it came in, but most 35mm cameras available by that point had easy load systems that largely eliminated the main advantage of APS. However, more than that, many of the key features were never really exploited, for various reasons. Mid roll rewind/reload - well that wasn't that big of a deal when there was such a paucity of film stock to choose from - aside from colour print film broadly in ISO 200 or 400, there was one black and white film, and one colour slide film - and both of those were not exactly easy to track down.
The smaller format was supposed to be made up for by new film technology, so that results would be as good as 35mm. But when that technology made it into 35mm film, the gap remained. The multi-format idea - well did anybody actually use APS-C? And plenty of 35mm cameras had masks that would crop to a pano format. I did use an EOS IX alongside my 35mm EOS cameras, and it was a nice camera - but limited with wideangle lenses, thanks to the smaller format, and limited in terms of film choice. Oh, and of course processing and printing was dearer than for 35mm... (I also own an IX7/IX Lite, an Ixus, and a Minolta Vectis S-1, but the IX was by far the best of the bunch.)
Nikon 1 - a nice idea, but not sure the technology of the time was quite ready for it.
Lytro - enough said.
Pentax Q - definitely the technology wasn't ready for this one. I've 2 Q bodies and a selection of lenses, I wanted to make this my travel system, but it really wasn't up to it. IQ just isn't there, particularly in low light - and for me, travel means shooting in churches and museums, at night, you name it.
So next a couple of contenders before my actual nomination:
Original 4/3 system - I never did get the point of this. Tiny (110 film sized) sensors in generally large (full frame/35mm) sized bodies, the acme of idiocy IMHO. MFT is just such a much more sensible use of sensors that size. The actual adoption of the original 4/3 standard speaks for itself.
Kodak SLR/C, and to a lesser extent the SLR/N and 14N. Kodak were of course dominant in DSLRs to start with, pretty much all of them aimed at the press market. Once their agreement with Canon finished, Canon worked very hard to catch up with the market, and launched quite a few key cameras. Kodak suddenly found themselves struggling to catch up themselves. Their partnership with Nikon allowed them to build several models based on Nikon film cameras, but IMHO they picked the wrong horse with the F80 (N80) instead of the F/N 100. But they couldn't do that with Canon, so instead launched the SLR/C based on a Sigma body - the SA-9, from memory. And the result is, well, something of a mess. Yes, it's full frame, and these cameras were designed to be upgradeable - but apart from a memory upgrade to the 14N, I don't believe anything ever was upgraded. The SLR/C was an ugly camera that feels pretty terrible in the hand until you get it nesting right,and even then... and the controls are incomprehensible to a Canon shooter. Results at low ISO are very good indeed, but start to fall apart pretty quickly after that. And at launch, it was up against the EOS 1Ds, and before long, it had the 5D to look ridiculous against. I do still use mine occasionally, but basically this was aarguably the camera that finished Kodak off as a serious camera maker.
And my winner is - not actually a camera. In fact it's not really a product at all, pure vapourware. Digital Film. A splendid idea, a way of adapting pretty much any 35mm film camera as a digital camera - but the problems ensured it never made it to market. This would have been brilliant if they could have got it to work when first announced, when digital cameras were still prohibitively expensive, and nothing like as well-featured as film cameras. And the idea of taking digital photos on an old classic - yes please. But it never became real, so for that reason, it has to be my contender for biggest flop of all time.
