The most important change in Gotham Knights is the appearance: the drawings are more manga-like. Therefore a great artistic link to TAS is lost.
Next to this we see that the old Robin has become Nightwing and there's a new Robin (a much younger one).
We also have Batgirl who joins the club.
Where Nightwing is a great character (well, it's bassicly Robin with more maturity) the new Robin is simply irritating and gets on my nerves
But I have to say, the show isn't a complete waste of time.
Simply for one reason: it's batman
How messed up can you make it? I don't care!
As long as we have a man, dressed as a bat who fights the ambassadors of evil, we'll watch it with a bag of popcorn.
But the popcorn tastes better while watching TAS!