Judging graphic design, just like art is very subjective but good graphic design most people can appreciate on some level. Their are no set rules on the artistic side of graphic design but remember graphic design is not art, its purpose is to communicate a message effectively. This message is usually always commercial so in this sense their are strong rules to obey.
Firstly, the intended message should be communicated in a way that is clear and easy to remember. It should attract the viewers attention, even when their are competing designs next to it.
Good graphic design should give most people a similar impression. This impression should assist the tone of the message being communicated.
Probably the most important characteristic of good graphic design is relevance. If the advertisement is not relevant to you in some way, why would you pay any attention to it?
Companies like McDonald's rely on great designers to create the impression that you are going to get a bargain when you eat there. Even though I don't eat at McDonald's often I can still remember some billboards advertising $1 french fries. Their ads are clear, relevant to their target audience and easy to remember.
Marketing text books have documented a phenomena called banner blindness. People are unconsciously ignoring advertisements because their are so many in the world. Making your ad stand out is a tougher job then ever, but it is now easier to find quality information on what makes an advertisement effective.
If you are not sure of the quality of a graphic design piece that was created for you, ask a few people in your target market what they think of the design when they first see it. The true judges of your advertisements are actually your customers, remember employees and friends may not always be the right people to ask.
To finish off, a video-based graphic design portfolio that I personally think is very good quality is below.