One of the world's largest social networking websites also covers a large category on Japan travel. Updated almost everyday with a wide variety of topics ranging from itinerary check to recommended accommodation.
World's largest travel site also has the largest forum on Japan with over 70,000 topics posted from all over the world. If you have a plan to go to a specific area, they have sub forums to meet the person with the similar interests easily.
Since they founded in 1996, japan-guide.com has played a main role to introduce Japanese things/places in English.
Although their Japan Question Forum is useful for people visiting (or planning to visit) Japan, their original contents covering the fundamentals on Japan are also useful before you actually visit.
The world's largest travel guide book publisher Lonely Planet also has a large forum for travelers to Japan. There are relatively many posts focusing on certain areas/activities rather than troubles/things.
Fodor's Travel is the oldest publisher of English language travel and tourism information in the US. They have launched a travel-related website in 1996, which has lots of features & trip ideas.
Their JAPAN forum is simply divided into two categories ACTIVITY questions & TRIP REPORTS and the feeds are compatible with RSS so that you can easily follow them.