Good question. I was thinking about getting the TIVO HD or 3, but decided to hold out, and stay with my cable DVR for now. Once the following is settled be either TIVO, or a competitor, I'll bite.
* CableCard 2.0 spec is finalized, and they have confirmed HW support for it (needed for VOD support, etc)
* TivoToGo functionality
* Copying between multiple boxes. This isn't a deal-breaker, as I have a media server ... but is a nice to have for stuff I don't really want to keep for long ... or if my server is running low on space or *no* down.
For TIVO itself, there are a few things I'd like to see added to their interface. Most obviously would be adding a 24hour skip to the program guide. Scanning throw to an upcoming day sucks.
An interesting, related question is that when they get CableCard 2.0 working, they could actually allow you to receive your provider's guide service. The question is, would they want to allow that? Probably not, since their main business model is to get a subscription. Then again, things change ... if everyone else does it ... they may have to. That said, I don't think they want to really get into the HW business as their model. With that in mind, I would suspect they would only support things such as VOD control unless absolutely necessary.