External SATA drive support and Tivo-to-Go. eSATA ports are present on the box but not enabled, so you can't easily add more show storage cap right now. TTG allows you to transfer shows to another player (like a portable). MRV is multi-room viewing so you can network the Tivos together.
Face it folks, these boxes were crippled and overpriced from the start. Don't be an early adopter of the S3, wait until they (or more likely, the cablecard folks) get a wakeup call for what consumers want with HD. Which is to say, everything they already HAVE today with SD. My 2 ReplayTV's run rings around the S3's in capability-wise EXCEPT for HD. But I'll admit that SD is slowly going away and HD recording is the future, so it's tough to sit on the sidelines.
So your choices right now are to sit this one out, wait for a decent cable or sat HD recorder, or build a Vista Home Premium or Myth PC. But you won't get encrypted HD channels with Myth, you have to have Cablecard for that (i.e. Vista, eventually).
The sat co's have come a long ways, I've heard good stuff on avsforum about the latest Dish (VIP622) and DirectTV (HR20) boxes. Tivo s/w is just around the corner on Comcast boxes, but they are going to charge extra (over and above the normal DVR fee) for the privilege.