Working with low-poly graphics and other size constraints like limiting texture size, as well as performance concerns like drawcalls, makes for a much tougher challenge than I would have anticipated. Then again, it is just how I like it.
I want to make sure the games we make run as well as possible on your systems. The closer we can get to the intended look for you, the better it is.
Since some people have asked, I will share some low-poly design philosophy insight:
Avoid squares and rectangles for building bases. This is dull and played out, unless you are recreating the dull and played out designs for ironic/sarcastic/comedic effect. Use more advanced quadrilaterals: En.wikipedia.org
Constantly ask yourself if you really need that triangle. Merge shapes that are too similar in angle when adjacent. Use triangular bars instead of square. For bigger cylinders depending on the size, use 5 sides if the object is smaller than the main character, 6 if it is equal or a bit bigger, 7 if it is bigger or a wheel on an otherwise simple object, and 8 if it is part of a larger structure.
Rejoice in the findings of the colonthree. :3
When making extrusions, remove any invisible backsides and disconnect the extrusion from the shape if it is a door, a window, a vent, a pipe or similar things. Then remove the face with the hole and make a new face behind it using only the triangles you need as there is no need for a hole if you can't pass through it.
Constantly reuse textures in new ways. Be analytic and creative with your UV-maps. A long board could be just a pixel wide on the texture, but if stretched it can fill a whole wall. This saves on texture space.
Never use more than one material per object, except for in very limited special cases.
Don't use transparency. This can be hard to design around but I like the challenge and the performance increase is worth it.
Never make too similar looking buildings. This is lazy, and makes for an uninspired setting. If you are going to reuse the same shape, make sure to do a very thorough texture swap so that at least it is coloured differently. You can also edit, add or remove minor parts of the building to make it more unique.
Solve complex geometry patterns using textures instead of polygons. Looking at N64 games has helped a lot on this.
Always look to improve what you have done as you learn as you go along. In my case, going back to objects I made earlier I can immediately tell where I dun goof'd and fix it. This goes for both models and textures.
And that's it! If there are any more questions please let me know in the comments below. :3