First, I would like to welcome you to the forum, I hope you stick around and see what we can learn from each other.
your question is an interesting one; I'm an American Graphic designer, and I did some casual observations of commercial design while I was there last year. I hope to do a more in-depth analysis in the near future. I'm afraid I couldn't tell you how Japanese ads differ from European ones, because I have seen very few European commercials. But I can tell you what I've observed in Japanese advertisements in my short time there.
One major thing I noticed was that the highest level of ad exposure was probably on the trains. As a student of the language, I found it a fun way to pass the time by reading and analyzing the ads hanging in the aisles and posted on the walls above the seats. Tokyo, having the highest population density, reflected the difficulties of grabbing peoples' attention in its advertisements. Primary palettes were the most popular; very high contrast and bright colors, with a lot of outlined text (meant to "pop" the characters). Space is money, so ads are cramped with information. This kind of ad was so prevalent, in fact, that the real attention-grabbing ads were the calmer ones with more white space (usually for big international companies that could afford it). Also, since it was summer, I found the ads with cooler palettes to be more refreshing and appealing.
I didn't pay as close attention to TV ads, but they varied depending on the product. A big difference I noticed was that they tended to use actual celebrities for the CM; American ads tend to have no-name actors in them, and I'm guessing European commercials are similar in that regard. Not very many cartoon ads that I noticed, but there were a few that used some sort of fictional non-human character alongside humans.
Another interesting tidbit that I noticed was that the train ads actually calmed down and became more relaxed as I moved west. The ads I saw in Kansai (in general, there were many exceptions) looked much more professional and, to me, were indicative of a clearly different demographic, and possibly a different school of thought regarding ad design.
That's all I can write for now, hope it helps a bit.