Wait, what's this? How come something so useful as delayed expansion (see previous posts here and here to see how useful it is) be evil? Well, it's not the feature itself that's evil, but rather the way you can end up spending a long time trying to figure out what's wrong if you make a simple, stupid mistake.
What mistake am I talking about? Consider this:
for /L %%i in (1,1,10) do (
set /A COUNT=%COUNT% + 1
Simple enough, you said. And you must have spotted the bug (if you have not, try to run it and see what happens). Now, of course if the loop body is much more complicated it would not be as easy. You see, the problem is that what you are doing is completely legal. Having %COUNT% in the loop body is fine as long as you don't expect it to be delay-expanded. It's a feature, but when you make that silly mistake (or more likely, someone else in your team), then it's not going to be pleasant to hunt the problem down.