Japanese arts are known for simplicity and minimalism. This does not translate to advertising.
One of the first things a person notices in Japan is the abundance of advertising in public places, especially in trains, at stations, and on the street. It is not necessarily limited to just major commercial districts; even residential areas are littered with pop-up banners and flyers.
The idea seems to be that if you're in enough places, eventually someone will notice you. Content should be created with this in mind.
Japanese advertising is a perfectly acceptable place for celebrities to appear, even A-list ones. Whereas Western advertising might use minor celebrities or relatively unknown actors, it is not seen as a step down when famous musicians and actors appear in an ad.
Japan is a top-down, collective culture where many opinions are deferred to figures who are perceived as a higher authority. Perhaps in line with this mentality, having a well-known public figure behind a product or service can be a powerful statement. Sometimes, the content is the almost entirely the featured person.
When speaking Japanese, it's normal to make points in an indirect and roundabout way. Advertising often follows suit. The point which a television commercial makes might be just a footnote at the end, while the run of the commercial focuses on something completely different.
For example, advertising for Softbank, a major mobile phone carrier, is well-known for the fictional White Family. Most television commercials and printed advertising involving these characters doesn't even feature a mobile phone. In the case of the former, dialogue might touch the subject of mobile phones but does not drive home any point. It is much more important to create a lasting impression in the mind of the observer.
There is no need to take things too seriously in Japanese advertising. It wouldn't be 100% correct to say that advertising content can be anything, but it's a pretty close assumption. Again, the idea is to create a lasting impression.
An extreme example of this would be 2009's Teinenpi Shoujo Haiji advertising campaign by Nissan and PLUS Heads. Even without subtitles, it the crudeness and unusuality of this commercial is obvious. Good luck forgetting what you just saw.