The dry galley is very important. I think that Steve Fredrick's technique is the most robust and effective way of sealing up the galley. It's the technique I used to build my trailer and it's also the technique used by Teardrops NW. Essentially, it gives you a double seal around the edges of the hatch:
www.teardropsnw.com
Insulation isn't as common as you'd think. In fact, I don't know that many of the popular manufacturers insulate their trailers. I insulated mine more as a weight savings (rigid foam being lighter than plywood), but it added quite a bit more work and I can't prove that I gained much other than 50 or so lbs of weight loss.
Timbren's axleless suspension is gaining in popularity, but I've heard a few stories about strange tire wear and a couple of frame failures where the suspension mounts. To be fair, I've also heard of failures with leaf spring stuff. The difference being that there are a lot more places that carry leaf springs vs the Timbren setup.
I'm a big fan of light colored interiors, lots of windows and open storage spaces. There are lots of manufacturers building with cabinets and drawers, but I feel that invariably I'll be trying to fit something into a pre-made space and it just won't fit. I purposely built my trailer with a lot of open spaces that will allow me to build to suite my needs as I figure out just what my needs are.
In my opinion, the best hitch to build onto the trailer is a 2" receiver that allows the end user to put on whatever it is that they want.
Outside color doesn't really matter to me so long as it's well done. I'm happy with the mill scale finish of aluminum, but I know that there are lots of different colors out there on the road. I'm not a huge fan of graphics unless they're subtle.
@flipmachine, for example, builds a beautiful trailer but I'm not personally a fan of his graphics. I feel that they're too busy (sorry Flip!). I think his current trailer would look awesome in the current Toyota Tacoma's Cement or Sandstone color. There are a couple of Subaru colors out there right now that would look great as well (Cool Gray Khaki, for example).
I'm a big fan of simplicity. I have an electrical system and a propane system and that's about it. No plumbing, as I don't want to have to deal with pumps, hoses or tanks (grey or freshwater). No solar as the trailer charges from the tow vehicle when driving and I don't feel like chasing the sun with the panels to get the most out of them when camping. Plus I tend to camp in the trees if possible to help keep things cool.
Also, I'm a big fan of curves but I also see the appeal of angular lines. I built my trailer curvy because that's what I wanted. I would have had more space in the box if I had gone with angles. In reality, I would have gotten the most bang for the buck by just buying an enclosed cargo trailer and converting it to a camper. I like having a trailer that I don't mind looking at.