The 921's programming is channel/date/time based. If you want to record ER you have to tell it the date, the time and channel to record and it does it. But if NBC doesn't show ER on that date/time it will record what's there anyway.
With the DirecTivo unit it comes with the Tivo software which besides being able to record like the 921 it can use name based recording. Using ER as an example again, you just tell it that you want to record ER, if you want all ER's or only new ER's and it records them. So if NBC does a special two hour ER the Tivo software will pick up the chanage and automatically record it.
One thing that the DirecTivo box may also do is be able to record OTA HD channels by name also. I think that's what others have said but I haven't read through the complete instruction manual to verify.
E* has mentioned that they might add name based recording but based on E*'s past track record of keeping promises I'd have to say don't believe it until it actually is on the 921. (I'm not bashing E*, just stating a fact, where's OpenTV on the 6000, Internet access on the 721, etc.).
The 921 also has a RF remote control vs.IR on the Tivo. I also think the 921 does on screen caller ID, the Tivo doesn't.