My #1 thing for me personally is the layout has to be user-friendly for me. I don't care about the "stuff" in the house, but if I ever decide to place households in those houses that are actually going to be incorporated into my story, I have to know that the house is at least route-able without me having to do a major reno on it.
I also don't worry about furniture style or placement of items until I'm actually playing in that house, then I decorate it to what I feel matches the sims who are living there.
Mostly for me, I need the exterior of the houses to be cohesive and match the vibe of the world/neighborhood so when my sim is out and about the aesthetics of the world make sense.
