I agree with ChrisFL that the Micechat board is one of the best places to get lots of detailed, invaluable information for planning a trip to TDR. But, I'd also like to post some info here since the Disboards are so busy and hopefully others who may not want to visit Micechat can get some info here. These are just my personal observations and how I planned our trip, so use whatever works for you!
One of the first things we did was determine the best time of year to visit TDR and Japan. This varies for everyone depending on your situation! For us, we are an adult couple and could go anytime. We're from Florida and didn't want to go when it was cold. We were looking for low crowds and decent weather. We looked at the historical weather for Japan as well as school calendars (TDR is nuts when school's out) and national holidays. We also looked at the calendars for the Disney hotels (Miracosta, etc.) - they list Value season, Peak season, etc. That helped us determine what weeks Disney was expecting lower crowds. We also looked at the list of scheduled closings for attractions. We ended up going the 3rd week of April...we missed TDL's anniversary and the cherry blossoms, unfortunately, but the crowd level was low and the weather was pretty good (60's during the day) and we were able to ride Pooh's Hunny Hunt as it was coming out of refurb.
Language barriers are small, both on and off TDR property. The Japanese are extremely helpful. Signage is very often in English and Japanese, not only at TDR, but also all around the country. The key is to learn a few key polite phrases in Japanese, like hello, please and thank you. Beyond that, you can rely on hand gestures (nice ones!) and pointing to get your idea across. Also, at TDR, there are often a few cast members who speak a little English...if a cast member doesn't understand you, they will run and get someone who might be able to communicate better with you.
Trip length - we did two days at each park and found it sufficient to see almost everything. I suggest no less than 3 days total, as ChrisFL recommended!
Cost - tickets are less expensive than WDW. Food prices seemed equivalent to WDW and DL, but the quality and selection are much better. Souvenirs are also priced similarly, but you will rarely see a t-shirt or some of the other types of souvenirs that are popular in the U.S. TDR caters to the locals and they like things like character ears, danglers for their cell phones and bags, and elaborately packaged food items (cookies, candies, etc.). We still found enough cool stuff to almost fill our extra suitcase, though!
Hotels - on property Disney-run hotels (there are 3) are extremely expensive! Even in the value times, I believe the cheapest Disney hotel is about 27,000 yen a night ($295 at today's exchange rate). There are 6 hotels at TDR that are "official" hotels and have bus service to a monorail station. They also sell park tickets in the lobby, which is convenient. The two most popular with Westerners are the Sheraton and Hilton. We stayed at the Sheraton. By pre-paying for our room, it cost us $170 a night. Our room was huge and had a view of the parks from our balcony. We just took the monorail to and from the parks.
By the way, the monorail is NOT free! You have to buy tickets. We got passes that allowed unlimited rides for a certain number of days. It was less about $20 each for about 6 days worth of unlimited rides.
There are also less expensive hotels in nearby neighborhoods. Some of these hotels are under $100 a night. You can take public transportation to the parks. Trains are inexpensive, frequent, and are without fail "on-time, every time". You can also do the taxi thing, too (more expensive!) If you want to stay more in central Tokyo and travel to TDR, there are tons of hotels there ranging from cheap capsule hotels and hostels to fancy five-star high rises...just be prepared for a travel time anywhere from 20 minutes to over an hour, depending on what neighborhood you're in and how many transfers you need to make on the trains.
Okay, I've babbled enough for now...hopefully the information is somewhat helpful. I've only been once, so others probably know more than me...but I did do about 6 months of research (daily!) so I "think" I know a little bit!
One more thing, the official Tokyo Disney Resort web site is very helpful, too...lots of info there:
http://www.tokyodisneyresort.co.jp/index_e.html