HD space is not a concern. Memory is. Both RAM and graphics memory.
Yup, I'm also pretty sure about that.
There are different types/looks of armor in the game already including different weapons/looks so using those is the ideal solution, they have to get loaded in memory anyway.
Adding different looks for every monster to increase randomness would more than likely make my 4GB of RAM look scared and worried.
I think that varying body sizes (length, width and height) would be the way forward, you would only need one model and skin for each monster and then a bit of code to morph the model within varying limits, not so much that the skin looks stretched, but enough to be noticeable, also it is likely that this wouldn't be too taxing on system requirements. Like some people said bulkier monsters could be stronger, taller monsters faster, it makes sense that some monsters would be stronger than others, some faster, etc, etc, but thats purely hypothetical.
In Diablo 1 and 2 every monster of the same class was uniform in appearance, apart from some palette changes, and it felt like they were generated purely as fodder for your character, which admittedly they were, but in the context of the story that doesn't seem right to me, those monsters would have existed long before your character walked on the screen, and surely, in my mind at least, some variation would occur throughout their lives, some would be very tall, some might be very fat, etc
I hear what people are saying about all of the monsters being slain very quickly by your character, and thats fair enough, but in diablo 2 you had enough time to notice different types of monsters, champions and uniques in the pack before you killed them, i'm sure you would notice different body shapes too, even if only subconsciously, its another thing thats adding life to the diablo universe. I wouldn't want this feature to be prioritized above any of the key stuff that makes Diablo special, to me it would just make the gaming experience a little richer, fuller and to some extent more varied (if stats related to body type were implemented).
If the devs chose not to implement any element of randomness for whatever reason (technical, time restraints, unneeded in their opinion) then I could understand but I personally hope that this is included in the final version. My two pence.
Some randomization would be cool, but too much and, as others said, it becomes to much work for the developers and takes up a lot of hardware resources. I do like the idea for size randomization and I think that is probably very doable without much of an impact on resources. I really don't think you need a ton of skins though - just a few for some enemies (zombies, in particular) would suffice, and others would need it even less. I like the idea of having different colored shields and whatnot to mix things up too - simple to do but adds good variation.
In terms of stats - slightly random hp would be fine, but I wouldn't change things too much. The "variety" in the actual gameplay should be coming from the different types of opponents, otherwise it would become a frustrating guessing game.