They don't have the bass impact and they definitely lack some fullness in the mids compared to LCD-2. They make up for their shortcomings in other areas though: better highs, bass quantity, soundstage, comfort. The mids are definitely TH-600's weakest link. Compared to LCD-2 where you can feel an acoustic guitar being plucked as well as hear it, TH-600 definitely feel a bit thin.
They're very good headphones though. I think they're more fun to listen to than LCD-2.
I actually think the TH-600 have more bass impact and quantity than the LCD-2. Not by much, but it is definitely there when I had my pair. It would stand to reason that they do being dynamic vs orthodynamic. I never found the TH-600 to be thin, just more airy in comparison. If you find them to be "thin" you might want to let your brain adjust to their signature before writing them off. Even though the headphones are fairly efficient, having good upstream gear will help them open up. They sounded noticeably fuller with a hybrid tube amplifier. On the WA7 I demo'd the mids were brought up giving them a very nice balanced sound.