1. This is a minor, annoying, me-being-persnickety comment: I like everything except for the major highlights, where the light source is not centered. SS Vanilla ships are generally lit in a centered way. Anyhow, that's a very minor gripe and I think it's kind of a wash; artistic license is important and if you feel that makes it, go for it
2. The only really major thing I see that I'd work on a bit more is the sense of scale. That's a fairly big ship for a Frigate, bigger than all of the Vanilla frigates.
To put it into perspective, it's only 8 pixels smaller in width than an Enforcer, and 18 pixels wider than the Brawler (16% wider, to put it another way), while being 4 pixels longer than a Vigilance. It's almost Destroyer-sized, basically.
However, it feels smaller than it should, because there aren't small, telltale areas of pure pixel-art work suggesting the true scale. A few specific greeble areas that really pop with high contrast would really make this piece perfect. I think that you could probably get where you want it to be in less than 30 minutes with some pixel-work bringing out a few things or adding a few new invented greebles that are small enough to reinforce that we're looking at something that's roughly 64 meters wide.
Anyhow, sorry if this sounds like, "hey, good work but it sucks"- it absolutely
doesn't and I think you will turn it into massive bit of Win, frankly; you've really gotten more progress in less time than most ever manage here and I am really excited about the design. Just think about scale a bit more and maybe find some areas to make that happen; get it to 100% awesome.