I think Tower Grove Park is a great St Louis park for pictures, especially when it's cool out. It's a little muggy on some hot July and August days. I particularly like the pavilion area by the children's playgrounds. It's on the center roundabout, kinda near the tennis courts.
What do I like about Tower Grove Park? It's normally close if I'm already in the St Louis area, or especially the South St Louis City. Lot's of trees. Neat pavilions; very ornate, colorful, big. And there's the pond by the ruins as well.
Any cons? Some other locations have better wind flow when it's muggy and hot. It's a little crowded around the pond with the ruins. But all said and done, it's one of my favorite locations if I'm in the area.
I see many wedding parties having pictures made in front of the Grand Basin, at the foot of Art Hill in St Louis's Forest Park. I rarely see people up on the hill. I've enjoyed making pictures on the hill and around the art museum itself.
It's a little difficult walking on the hill if you are in heels.
On a scale of 1 to 10, Washington University Campus is about a 10 for me. It's quite, cool (it's up on a hill and there's normally a nice breeze, shady, and has lot's of arches, cobblestones, etc. Also, it's pretty easy to get to from most places in St Louis, and right across the street from Forest Park.
I love shadows and directional light. It's more fun to work with than "open sky" type of light. Arch ways and overhands create directional light and can be used for dramatic pictures or just nice portraits. Arch ways and overhangs are in abundance on the Washington University campus. Which also makes it a nice rainy-day location. And you can get around without having to walk through the grass, which I'm sure makes it nice if it's been wet out.
One other thing; there is usually a restroom available on campus.
I've just started on this and hope to add more locations soon. Let me know if there are any particular locations you would like to see.