If I'm directly comparing them I would say something like this:
TH900 has a very natural, euphonious sound that is great with most music. It is detailed, but not as much as the LCD-3. It soundstage is wide, though not quite as large as the LCD-3. It is closed back though, which is of great benefit. It is drastically more comfortable. It doesn't have a detachable cable though. And maybe most importantly, you can drive it with almost anything and still get good results. It may be a little too sensitive in some ways though.
LCD3 is darker than the TH900, but more revealing (I consider this a wonderful anomaly). Both headphones are forgiving though. The LCD-3 is more picky regarding amplification and not nearly as comfortable. It is also open back.
When they are both amped to their best, sonically I think you're comparing a dark, but revealing and expressive sound-signature (LCD-3) to a euphonic yet smooth and sultry sound signature (TH900)