As mentioned above layers are a brilliant way to make a building look impressive. For those who dislike microblocks (or like me, find them to become laggy once used too often) look to vanilla items for usage. Stairs upside down make a good edge to a roof, fences can be used as thin wooden pillars, or as a wooden trellace up a building.
If you have a large building of one colour which looks a little imposing try adding leaf blocks running up "hidden" logs, one log can have leaf blocks coming off it for a good amount of blocks and is easy to hide either under the leaves themselves or within the wall. Make the leaves multi-layered to give a bit of depth and have them crawl up the side of the building. Cut out holes for windows within them and it will give both your windows a feature and make the house look alot more detailed with very little work.
Just because we have chisel, do not forget decorative blocks included in many of the mods, MFR black brick varients are too often ignored and even on default their texture is quite nice, even when used in large amounts. Another example of this would be maricultures limestone bricks, both the large and smaller versions have very clean and detailed textures that look good even on default, it makes a brilliant alternative to marble when making Georgian/Victorian style buildings.
Thaumcraft candles look amazing placed on tables around a fantasy styled house.
If using default texture pack, avoid large use of minecraft "brick" blocks, when used too much they will confuse your eyes causing the building to be almost painful to look at. Break this up with the leaf method above and some wood and the building will retain its steampunk/industrial theme without looking too "busy" to the eye.
Railcraft smokers are woefully under used. Use them more!
When designing an interior do not miss out on various features even if they serve no purpose, fireplaces, bathrooms, carpets ect all add to the "wow" factor of a building and quite often can be made into functional things through creative use of mods. Fireplace concealing Smelters can serve as a place for cooking your foods!
Pillers can be inside too. Break up your walls with supportive pillars. Same applies to roof spaces, use beams of logs going in a criss cross pattern across the top of your walls then build the roof ontop of them, leave the gaps inbetween empty and youl have an impressive roofspace.
Chisel those bookcases! All "chisel" bookcase styles have a connected textures, allowing you to mix and match them to create some pretty impressive librarys.
Design your buildings with a 2 block high "space" between floors. This allows you to run pipes and wires out of sight, and keep your areas uncluttered with messy conduits.
Buildcraft Floodgates will pump more than just water...Any liquid in the ore dictionary is possible. Lava lakes, glowstone lakes, rivers of AM2 Liquid essence...you name it.
If you have the statue mod, glowstone retains its light level even when made into a statue.
thats all off the top of my head, once my servers back up i might come back with some pics and tips
