David and Donna were great in the high school years, but I think trapping them in that relationship (with a will they/won't they thing) really hurt their characterization in the College seasons.
David clearly evolved more in season 6 when he was with Valerie, and yet they still have him pining for Donna by the end of the season without any build-up to it.
I do suspect that his real-life break-up with Tiffani might have had some impact on the decision to drop David/Val
