Although I agree with Jurek7 that genetics might not say a lot about the individual habitat adaption of the different subspecies, I'd be careful to use just phaenotypical characteristics as crucial criteria. Remember all those "species" that were described by early scientists/naturalists that turned out to be more or less irregular specimens of the same species (or, like eclectus parrots, just of different gender)? Or think about the high variety of colour and pattern morphs within a single species, for example, Dendrobates auratus.
About the characteristics of Caspian tigers:
-Third largest tiger subspecies (overall body length 240-290 cm, weight 90-220kg)
-light coloured striping
-dense fur with a belly and a nap mane
Contemporary observers stated a clear difference from the other subspecies; yet some of today's tigers display similar characteristics.
http://www.tiergarten.com/fotos/thumbs/52187.jpg
Due to neglect of subspecies seperation in the past and thus rather 'wild' breeding, quite a bunch of the tigers (especially Amur) kept in zoos might not be "pure", subspecies-wise, and therefore differ from their wild conspecifics...
The article states that Caspian and Amur-tigers seemed to have intermingled in several places of their distribution even until recent times. The segue population appears to have been wiped out by humans; therefore, it is not clear whether they were/are really 100% different subspecies, or just slightly different local forms of the same subspecies. Personally, I'm looking forward to the future results involving the other subspecies.
@Hix: Although I haven't given hope all hope on surviving Caspian tiger, I'm unsure whether most sightings by soldiers are valid. Like the greatest part of modern society, most soldiers aren't skilled in animal identification, which might result in exaggerated misinterpretations of unfamiliar species (see ABCs, Người Rừng...).