What happens when a bike has not had maintenance for a long time? I recently traded my Kent BMX bike I got from Wal-Mart: 

for a Ross road bike ). 

My friend tells me the bike sat in his back yard collecting rust, I see it has rust on the chain, the gears, brake lines and the chrome handlebars. The bike rides ok but the gear shifts need a good push and pull with my thumb to work. I know the bike has had no maintenance in a very long time so I was wondering what should I do?
When I was doing bike rehab for Christmas Anonymous I saw many bikes like this.  It can be good or it can be bad, depending on how much weather it's seen.

First thing is to do the obvious -- wash it (we used a power washer), clean &amp; oil the chain, clean and oil the derailers.

If it's been in the weather enough then the cables will rusted solid and will need to be replaced.  Otherwise, squirt spray lube (not WD-40, but something that's more of a lube) into the ends of the cables and work them well.  Also lube the brake lever pivots and put a dash of lube on the brake caliper pivots (but take care to keep oil away from the brake pads).

The bearings are the biggie.  From the sound of it yours may be OK, but exposure to the weather for years contaminates the lube and washes some of it away, so it's best to repack all the bearings, starting with the wheel bearings.  (I've seen bearings where all the lube was washed out and there was nothing but bare (and rusted) metal.)  If you've never done this before, start with the front wheel, after studying instructions on the Park Tool site.  And if, when you open up the front hub, the bearings look good and the grease appears reasonably fresh (it will always be dirty) then you can defer work on the other bearings.

Another thing to look for would be dry-rot with the tires and tubes. Cracking in the tires is a bad sign. Be ready to replace the rim tape if you have to change tubes.