Here is an article that gathers together some of the mounting academic evidence for something that should have been pretty obvious to begin with: that kids should play. According to the cited studies, playtime serves a very important role in children's intellectual and emotional development. If this is so, then it is a grave error when we make preschools look more and more like middle school, forcing children to sit and do schoolwork for hours every day.
I think my family has been lucky with preschool options for our two boys. At their tender age they've already attended four different schools on two continents, each with a slightly different teaching philosophy. But all have focused mainly on play, gradually introducing more structured activities as kids get older. It seems like drawing and other manual crafts and arts have served as a bridge to slowly incorporate more intentionality and order to kids while still leaving them free to explore and experiment. Only relatively recently has my older son started to do much in the way of practicing letters and numbers on worksheets, and even that occupies only part of the morning, with more playful and artistic activities occupying the whole afternoon. I feel like this is just right for him as he approaches five years of age. Our littler guy is still mainly doing playtime and painting and storytime, which is just right for him.