This is kind of a silly one, but it was a concious decision - and people often seem to think it's just a alck of furniture. Basically, most people have, somewhere in their apt, some chairs and a table. I was thinking about it after I moved the last time, and I realized that I really like just being on the floor. I also recalled that most of my friends usually sit on the floor anyway, so I decided I would own no high seating.
Now, the obvious problem with this (and if it's not obvious it soon will be if you try it) is back problems. If you just sit on the floor all of the time your back will almost certainly come out hurting somewhat. Well, I have 2 bean-bag chairs, which I got free a little while ago. These allow people whose backs are sore (or me when I'm reading a book for a long time, or working on a computer) to sit back with some back support, while at the same time remaining at the same height as anyone on the floor. This actually works out wonderfully in most situations.
My apt is carpeted. I would prefer to not have carpeting, but without it I would almost certainly require padding of various sorts for the floors - possibly I would've tried the japanese sleeping mats. In any case, this also means that the floor is not a flat surface for some purposes, thus you need a true flat surface. This is probably true anyway in the end since almost no flooring is ever flat enough - and it tends to be a bit dirty. So, I have a coffee table. This keeps the height reasonable, and gives me a place to put a laptop, a bowl of pasta, or a cup of juice.
For several months I actually had no table, and just had the two bean-bags. I picked up my futon though (see sleeping ), which techincally adds to my seating. This remained in my bedroom for quite a while after I acquired it. Later on I realized that distributing my 2 pieces of furniture between 2 rooms was silly, so I moved into my livingroom and now live in only one room of my apt. This means that when I have guests over I can fold up the futon and I actually DO have seating. It's still far lower than a normal chair though. So I'm lying about not having seating at this point. Still, the experiment remained for a while. I WANTED a futon that was basically on the floor, but had a frame so when it became a couch it was basically just a cushion on the floor - but the model I wanted went out of production :(. Still, what I have is fairly low and not too bad.
Works fine, I never understood chairs that well. They are great for sitting in one place doing work on a computer or at a desk for hours. I do not do this though when I am at home, since I have a regular desk job currently - so why have chairs at home? I could use a larger coffee table if I wanted to have a dinner party, but that's about the only issue. If I had friends who had certain health issues it might also be problematic, but this has not come up. All in all, I like the setup.