Television shows (and many, many, many other things, but I'm talking about TV just now) are judged on two separate scales.
One is the "What" scale. What is the show doing/trying to do? Is that thing new, exciting, original, interesting in and of itself. Is the concept itself a great thing? If the concept is great, than the show itself has the potential to be great. Not very good, but actually great. Lost is a good example of this.
The other is the "How" scale. How well does the show execute its concept/grand idea. Since most shows don't have a great or original concept, most shows are judged on the "How" more than anything else. A show CAN be great by execution alone. Homicide: Life on the Street had a concept that was basically quite similar to many shows before and since, but the execution was so astounding, that it achieved greatness.
After three episodes, Grey's Anatomy isn't great. It has a fairly basic concept ("like Scrubs, but not a comedy", or "like ER, but with surgery interns and residents"). But it does have very, very good execution. It has some of the funny, it has a really good cast, the writing is good, and manages to avoid going over the top when that's a danger. It has a superb lead in Ellen Pompeo, a really nice ensemble cast highlighted by Sandra Oh and Patrick Dempsey (yes, THAT Patrick Dempsey. he's returned from the Eighties, and now he can act), and it looks pretty good.
I stopped watching ER after a season or two, because they went too hard for the high drama. I'm hoping Grey's Anatomy doesn't go that route. It had a solid start (and is doing real well in the ratings), so there's hope.