One of the first problems that glass is smudges. Kids always go
to put nose and fingers -- producing some of the worse streaks and marks
-- I would REALLY hate to be a docent and run around cleaning glass
all day. But smuges fall into the category of "you can't do anything
about it" ---- well, except aim the camera above them (or learn how
to Photoshop out the smudges. I tried that once and it was A LOT of
work!). LeLe is a picture w/ the smudges -- w/ a lot of the picture
cropped because of them (and bad angles due to lots of kids around)
The second problem is lighting. If there's glass keeping something
in, the exhibit is usually inside -- no optimal "sunlight" to light
the shot. That means the use of the flash. And that means
a blinding white spot in the picture.
How do you get rid of the flash issue?
The answer is simple (providing you have room and not surrounded
by rug-rats) --- stand at an angle. Never shoot anything
directly in front of you. You'll just end up with a big
white spot instead of the object of your desire. Angle about
45 degrees - line up your shot - focus - shoot. The flash
should be off towards one side or another, in the glass, and not
directly on the object you're trying to get a picture of.
So glass IS a doable when taking pics. Not optimal - but
you've gotta take what you can get sometimes!
This beautiful kiwi colored bird was shot through glass:

There's almost no evidence that is what the divider was between
her and I.