Things I have been learning about in Uni that are considered essential for working in env/char/prop - Texel Density. It's not so much the render cost of the texture, but the effective usage and scale of the UV map and texture - a good tutorial is: 
Figuring out Textel Density
Figuring out Textel Density
It's something SL struggles with - and it's something I have not done well in the past myself
I think that SL unfortunately is an unideal situation to try to conform to this, as customers are used to... well, obscene texture use in items that don't call for it. Some brands have dozens (!) of textures on a single medium size furnishing, not to mention the material maps on top of that. So if you try to make sanely textured things...
People aren't used to it, and they'll snub it. It's frustrating for sure, since a lot of these things simply don't call for the textures they get, they could have been more efficiently designed in so many ways.
It doesn't help that SL will reload each texture per face, even if they're the same UUID.