You get to watch pretty much everything on ABC, NBC, and Fox. CBS doesn't have an agreement with them, so those shows aren't online. You can also watch past seasons.
Netflix, however, rarely has shows that are currently on air--it's a great thing to do rather than to rent movies, but it doesn't replace live television. It used to have a much larger selection of movies online, but it cut back.
To replace TV, I'd go Hulu. However, perhaps the worst thing (for my fiance, anyway) is that you can't watch tons of live sports.