my favorite part is seeing everything at the end. (not necessarily in a picture since I have such a hard time taking them) Every room seems to be a challenge for me these days and sometimes it takes me hours or days to finish one room and weeks to finish a house. But in the end when I see my sim using everything and moving throughout the house with a high environment score, it really makes my day.
I also really like the ability to express the different styles and personalities of my sims in their individual spaces. In a sense I am sure that these styles are maybe my own very weird tastes, but in the sims it is free and I can try so many different things without spending any money.
Another thing I like is trying to figure out how I can work with the space in a room. sometimes rooms are very small or large or have weird shapes, etc. so it is always an interesting challenge to make everything work and look nice when the spaces are hard to decorate.
so yeah....enough of my ramblings