1. Make sure it's playable.
2. If you are unsure, get inspiration from real buildings and interior designs - Google is your friend.
3. Don't be afraid of using MOO (as long as you play test your entire build after it's done before you share it and make sure it works)
4. Learn from what other players have done and "take" ideas to your own works. (As long as you aren't flat out stealing.. in other words uploading someone else's work that you've changed a tiny bit - that's a huge no no!) Challenges are an exception as you have to upload the challenge host's creation but those always have proper tags.
5. Take a look at building tutorials - i.e. Kate Emerald has done amazing tutorials!
6. Don't make massive rooms and leave them half empty (i.e. there's no point having a room that's say 10x14 squares, if it only has a bed and a wardrobe) In sims 4 the sims can function is way much less space than in any previous sims games! It's a trial and error to learn.
7. Enjoy what you do, it's supposed to be fun after all :D
Builds I'm proudest of (as I can't possibly choose just 1):
Marshal Hall Is actually my 2nd build in sims 4 when I only had the base game without any packs.
Don't have a picture in imgur.
Eternal Gardens Was experimenting with terrain tools, pools and jets.
https://i.imgur.com/ShYhfez.jpg
Mermaid's Call Was personally a huge challenge for me - spent a lot of time researching old ships.
Don't have a picture in imgur.
Willow Creek Greenhouse A mammoth build for me that I was very proud of.
https://i.imgur.com/dWZKaWj.jpg
Gift - B&W Modern The very first modern build I've ever done. I used to absolutely hate anything modern but I learned to love it.
https://i.imgur.com/CumVWsH.jpg
Blue Cup Café and Club 69 My very first industrial style build. Spend a lot hours building this and detailing the landscape (there's a murder scene at the back, lol)
Don't have a picture in imgur.
As for rooms, this is the 1 I'm most proud of:
Pond Took hours to place all the plants and flowers <3
https://i.imgur.com/zD97ZkA.jpg