Although it's kind of embarassing to admit, the dress-up doll type play and content creation offered by the Sims is one of its strongest elements. The Sims 2 Body Shop, along with any decent image editor (Gimp 2 for me), make it relatively easy to edit some of the standard clothing or create relatively novel clothing from scratch (the 3d geometry doesn't yet have a good editor).
A lot of the clothing is skin tight, or almost so, which makes things simple- there's no extra geometry or complicated folds. There's a few clothing models that have jackets, open on the front, with shirts underneath- but the jacket and shirt geometry are welded together. It would be interesting to see a more complex but freedom enhancing clothing model - I should be able to select a shirt, and then a jacket, and see the shirt underneath, and the same goes for underwear with revealing clothing (low-riding pants, or whatever). I can and did do the same thing with image editing, but this is cumbersome.
For complicated 3d geometry it could be error prone - protudences from what is supposed to be a underlying garment may stick through the outer garment, but that could be avoided by procedure if not programmatically in the game. ['protrudences' only has 51 hits in google- is it a word?]
An annoyance related to the above is the separation of full-body outfits from tops and bottoms- why is a two-piece swimsuit a full-body outfit? It would offer more freedom to allow mixing and matching there.
I'm yet to discover a way to convert a top to a body outfit/part, which would address the above problem partially, despite a few token efforts at file renaming (the .bmps exported are preceded with top~ or body~) and hex editing (the .package file refers to the file names it expects to find for the .bmps). Those efforts result in just default clothing getting loaded when I attempted to load it.
The other disappointment is the huge contrast is the customizability of the faces versus the bodies- there's just the adult, teen, child, & baby types in male and female versions, with slim and chubby types, and then different textures for different levels of fitness. There's dozens of slider bars for the faces but there's none to change the bodies- all the sims seem exactly alike, not even their heights apparently.
It would be interesting to have a clothing model where the sims react to each other differently depending on their outfits- the game could process the alpha maps and see how much of them are transparent (for a 'revealingness' or skimpyness factor), and also depend on clothing metadata (what type is it- but it probably does some of that already).