We've finished 2 basements, both >2,000 sq.ft., one was a walkout.
Not sure of the climate where you live, but the number one thing we did in both that was the absolutely best idea was putting in a dedicated HVAC system for the basement (separate furnace and A/C). All too often the basement is cold vs. the upstairs, and this solves the problem perfectly, and when we have parties, it can keep the place from overheating with all the people. And we also zoned those systems (very similar rooms in both houses) so we had 3 zones for the basement: family room, 2 bedrooms, and home theater. With zoning, we only need to heat/cool the rooms we are in, and it keeps each area at the perfect comfort level. It is also very nice to, for example, heat up just the theater room in about 10 minutes when we want to watch a movie at night in the winter.
The one thing I wish we would have done differently is to not include a tub/shower in the bathroom(s) -- I wish I would have just gone with a nice shower. Tubs are mostly for little kids, and generally you wouldn't have your little kids in the basement bedrooms, so no need for tubs.
I don't like cold, hard floors, so we had good carpet with high-quality padding for the living areas of the basement which makes it very comfy to walk around w/ socks. We had tile for bar, bathrooms, etc.
In our current house, we put in a great, good-sized bar which we really like. It has been great for parties and entertaining. The only thing I would have done different is add the ability to integrate a flat-screen TV to give it more of a sports bar feel. We included an apartment-sized dishwasher (18" wide vs. the standard 24") so we don't have to lug glasses and dishes up and down the stairs to clean. Under counter fridge and separate wine cooler round out the bar.
Kurt