I have only had vintage bikes, but I will contribute what I know.

If you are looking for your first bike, go find your local bicycle co-op. Have them teach you what they know and show you what they have for sale. Buy your first bike from them, because it will be cheap, fully functioning, and easy to replace if you mess up while learning something on it.

Never miss an opportunity to talk to your friends about the benefits of cycling to self and community. Make sure to point them in the direction of your/their local bicycle co-op.

If you KNOW something is very wrong with your bike, do not go on a 70 mile+ adventure.

If you are going to buy your main and only bike, get one with some big gears. Do not buy a fixie unless you know your commute is flat, and thats all you will use it for. Do not buy a beach cruiser pretty much ever. Do not buy a mountain bike unless you commute/excersize/adventure off-road. Big gears means something bigger than 24 on the largest rear cog, and the average 52 & 42 on the front. Get a triple crank if you are really concerned about hills, or are out of shape.

If you buy a vintage/classic bike, watch out for paint coming off when you swap parts or park it on a metal bicycle rack.

Dont buy overpriced junk, and look over all the parts on said junk carefully to make sure none are bent or broken. The top tube on my 77 Raleigh Grand Prix was bent several centimeters to the right. Lesson learned, I bought a new 83 Raleigh Olympian.

If you are looking for older bikes, make sure to look for mid-range to upper-range frames with 700c wheels. Anything mid-range will ride significantly better than anything thats cheapo. Riding on 27's is definetely doable, but riding on 700's is even better because you can buy them anywhere if you get in a jam.

Dont buy a bike that has shifters on the handle bar stem, or mod them out after.