Glassware
While traveling in Germany, I was quite impressed by the fact that each beer served at a cocktail lounge had its own specific glass it was served in. So if a lounge had 5 different beers on tap, they also had 5 different glasses that they had to have on hand to serve them in. And these glasses didn't just have different "logos" on them but each had a unique shape and design as well.

This might be a little overboard for a home bar, or even a traditional American bar as well. But it shows the level of importance that having the proper glassware can have.

Depending on the types of drinks you find yourself serving, I think the following types of glassware are virtually required to have on hand:

Cocktail Also commonly called a Martini Glass, but is the essential glassware for any true cocktail

Old Fashioned Sometimes called a "rocks" glass. This is the common vessel for a cocktail that is ordered "on the rocks"

Wine You can drive yourself crazy getting just the right glass for a particular variety, or even vintage of wine. But unless you really have the room for storing a lot of glasses, just some nice simple wine glasses will be fine. However if you do want to get a little fancy, try to find glasses that are relatively thin, and have crisp and uniform edges.

Beer Also called a Pint glass. Not only are these good for serving pints of beer in, but sodas, as well as drinks that include a lot of crushed ice go well in these.