See I disagree here. In Buffy, Angel and Firefly I straight away liked the characters and knew what they were about. Dollhouse I liked one or two and found a lot of the rest dull, that never changed throughout the 2 seasons.
Im not sold on the characters in SHIELD at all. The only interesting one to me was Coulson and thats because he's already established. (Yes I just watched the pilot today, I wanted to be able to see if Channel 4 cuts anything out)
Im not saying that the characters wont grow on me and of course as with a lot of shows actors will grow into their roles and iron out problems but I just think with Whedon stuff they are usually a lot better from the get go.