There is a definite difference between then and now.
One particular character who has changed a lot, and not for the better, is Marge. Watch a season 4-7 episode and then compare it to a season 17-20 episode. There's much less intensity, speed, and range to her voice. You won't hear anything like her chewing out the family at the end of "Scenes From the Class Struggle in Springfield" nowadays, or trying to weasel her way out of talking about her father in "Fear of Flying". It just doesn't happen. The exception is the movie, but even there, the only bit that really stood out to me was her tape to Homer. Whether this is simply because of Kavner's vocal inflections changing over the years, or the directors who don't know how to direct her, or the writing not giving her much to work with, that's unknown (personally, I think it's all three). But there is a difference.
Another is Skinner. We never hear him raise his voice much anymore. Even in an episode like "The DeBarted", when he threatens the school assembly after being pranked, it just sounds flat. You don't feel threatened by the guy at all. Compare that to "Whacking Day" when he expels Bart, or "Bart the Murderer" when the goons corner him in his office and he remains indignant, or heck, even a Scully offering like "Brother's Little Helper" when he yells "BART SIMPSON!" after Bart floods the gym. In those instances, you can feel the anger.
Lisa's voice is different, too. While it hasn't gotten bad, per se, there is a difference if you compare an episode from seasons 5-8 with something now. Heck, even the jump from season 8 to season 9 yielded a difference in line reads!