Crank,
I think there's several issues here.
Firstly the type of feedback you're getting might not be that helpful. Some people are quite inarticulate when it comes to giving critique on a work. How often have you seen posts (on other sites, not this glorious corner of the net obviously) where people simply type a negative comment with very little explanation of their thoughts behind it. Or conversely a positive comment like 'wow', 'beautiful' or my personal favourite 'nice'. While it is flattering to receive any comments by other people on our work and especially positive ones I find it often very limited in terms of practical use to my development as an artist. The majority of people who leave such comments are not doing it to be purposefully vague or obtuse but are normally somewhat uneducated in how to articulate an opinion to provide the kind of help we are looking for as developing artists. It is perhaps due to our nature as visual animals that we feel more inclined to pass judgement on the visual arts than to do so on literature or poetry.
Design is a complicated issue and projecting our vision of how future design may be influenced is even trickier. As I'm sure you're aware design has to work. There exists a more practical framework around which we can establish 'rules' to judge an item as well or poorly designed. When designing anything, whether for immediate manufacture or as a vision of what may possibly come, good design should always ask 'how'. How would this be constructed? How do I articulate this particular item in this particular way? How does this spider mech function? A lecturer of mine once told me that the difference between a designer and an artist is the difference between How and Why (a little simplistic I know but then he was a craftsman and believed his job was to bridge this difference).
I guess your job, in working up this spider mech is to answer both the how and the why. Perhaps go back to the original posters of the critique and ask for more clarification.
I guess the biggest issue here though is how you and the critics view your image. A lot of people would view your current work as a finished piece, the culmination of all your study and skill up to this point. This, I would guess, is not how you would view it. While partially true I would assume you also see it as a progression and the jumping off point for your future work, an aside which, while you hope will stand alone as a work of its own merit, is more about your development than your destination.
I guess what I'm saying is while some people will judge your image based on the design some will judge it on how you articulate your ideas visually, try to decipher who's doing what. Some people will leave you considered opinion, some will go "Wow, oooooh, ahhh!" or "uuugh! crap" and some will have their own agenda for posting a comment. Learn from the first, consider the second then forget about it and wipe your backside on the last. Some people will misunderstand your intention, try to consider how they view your work but dont get hung up on it.
Your making good progress so keep going. After all we're not painting the Sistine Chapel here, we're laying the foundations upon which we build the walls, rest the ceiling and finally paint our masterwork.
Whew, that was longer than I intended but I hope it helps. I need to lie down for a bit