Well you need all the usual furniture:
Dinner table, chairs, desks, lamps, shelves, night stands, coffee table, bed, and so on.
You need lots of couch space, particularly if you have friends crash at your place.
You definitely need a good TV and stereo system. Between my roommate and I, we had an entertainment center with a good selection of consoles, his bitchin' stereo, and my massive HDTV.
Might I also suggest an apartment name and theme? Being avid Fallout fans, we dubbed ours "The Fallout Shelter." I put up a bunch of Cold War paraphernalia as decorations ("Fallout Shelter" signs, Civil Defense dosimeters, survival manuals, duck and cover posters, gas masks, etc.) My friends' apartment was "Team Roommate." Two of them are into martial arts, so they have a bunch of related decorations (weapons, sparring gear, Bruce Lee posters, etc.)
We also had our fearsome "gun closet," but understandably not everyone has or wants one of those.
The other idea for our bachelor pad that we came up with was the "trophy wall." On it, we put all sorts of random things that we'd gathered in the course of our adventures. Among the things we had: .50 cal bullets from a disused military range we sneaked onto, cartridges from all the ammo types we'd ever fired, a weird piece of knotted wood from one of our camping expeditions, a map pinpointing the places we'd been in our state (the acquisition of the map itself had a story), a stuffed animal I won at one of those grabber arcade games (on my first try, I can boast a 100% success rate.) It's a conversation piece, if you have visitors.
You also need plants. Don't know why, but you need to have at least one, and keep it alive.
Alex_P said:
You absolutely need a clean bathroom. Dirty bathrooms say "I have poor personal hygiene!" Would you want to sleep with someone who had weird gunk growing on her toilet?
Can't agree more. Same goes for the kitchen, which I was admittedly lax about. Do keep your fridge well-stocked with things you'll eat, and not just booze. If you want, you can do what my sister did (she's a bartender) and set up a shelf especially for booze. You can decorate it with a neon 'bar' sign or something.