USJ is similar in layout to USF. Imagine USF with the biggest crowds you can possibly imagine, and you're close.
Do NOT visit without purchasing Express Passes in advance, and do not go on a weekend. Express Passes will not be available the day you go (they always sell out) and lines for most attractions will range from 2-5 hours.
You can only buy tickets and express passes in advance via the website, which is in Japanese. This guide is invaluable and the only way to do it:
http://www.dondelaofertamelleve.com/p/how-to-buy-usj-tickets-online.html
If you visit during Halloween Horror Nights and want to try their mazes or during Universal Cool Japan (spring special event focusing on anime properties) buy Express Passes for those as well. You will not be able to do much, if anything, by waiting in regular lines.
Imagine the most crowded theme park you've ever been in, and then multiply that by 2 or 3 times. That's an average day at USJ.
Several attractions are direct clones and you can skip them to save time: Spider-Man, Jurassic Park, T2:3D, WaterWorld, Shrek 4D, Minion Mayhem.
Space Fantasy, The Flying Dinosaur, Universal Wonderland (kiddy rides themed to Snoopy, Sesame Street and Hello Kitty) and Hollywood Dream are unique to USJ and all worth seeing. Space Fantasy in particular is incredible. Backdraft and JAWS are clones of the now-closed attractions in the US, and are now exclusive to USJ.
Harry Potter is a direct clone of Hogsmeade at USH, and extremely similar to IOA's. Forbidden Journey is in 3D, a change that does not enhance the ride. Your Express Passes will likely include this ride, so you might as well ride it, but if you miss it, you didn't miss much.