/console set extshadowquality 5
0 through 5 range. 0 off, 5 highest quality.
If your video card doesn't support them, I doubt this will do anything, though. Have you tried updating your drivers? Mine did not support the new shadows when they first came out. After a couple driver updates it now does.
If you like the old style shadows back from when WoW first came out (just a circle):
/console set shadowlod 1
0 off, 1 on
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