I think spinach is number 3 or 4 on the WFM ANDI list.

This is a good example of the principle that variety promotes good nutrition. Greens are excellent foods, and they come in many forms -- and from several food families. Eat greens often -- raw, juiced, cooked. Try kale, chard, collards, mustard, bok choy.. all the leafy greens are amazing foods!
Most kinds of lettuce can be cooked like greens, but do not have the levels of nutrients that dark leafy greens do.

While it's true oxalates in spinach can reduce calcium absorption, I haven't seen any scientific studies showing cooking affects the process.

On the other hand, studies have shown cooking vegetables increases phytochemical availability and antioxidant capacities.

http://pubs.acs.org/doi/abs/10.1021/jf072304b

In any case, spinach is a good source of beta carotene, vitamin B6, folic acid, potassium, zinc, magnesium, dietary fiber and a handful of phytonutrients.

My understanding is that spinach is "healthiest" when eaten raw or barely cooked, because the calcium and iron (and maybe other nutrients) in spinach get bound by the oxalic acid and therefore unable to be absorbed by the body.

The warning above about reheating spinach piqued my curiosity. Checking, here's an explanation, written by a Ph.D. in nutritional science:

http://www.utsandiego.com/news/2009/sep/23/fear-reheated-spinach-seems-unfounded/

I'm still curious about why one might not want to eat it too often.

Spinach has a multitude of health benefits. It is also one of the vegetables with the most pesticide so it works to buy organic. Eat it often and enjoy!

But don't eat it too often and don't reheat cooked spinach, not including the frozen kind obviously...

Yes! It's extremely healthful - dark leafy greens are very vitamin-packed! Something to eat lots of!