My number one suggestion would be to learn when/how to balance the light a bit better. The how is easy...your flash exposure was pretty spot on in all of them, so all you need to do is bring up your ambient exposure. It's as easy as dropping your shutter speed until the ambient is a little closer to the flash.
Now for specifics:
#1, the groom is a tad overexposed and doesn't appear in focus. This is the best "balanced" exposure here, but dialing your flash down a bit would have improved the lighting. The focus is another matter.
#2 has the opposite problem - not enough fill flash on the shadow sides of their faces.
#4 is very nice
#5 is the worst offender with the background so dark that it doesn't look natural. Looks like a fake backdrop (as trantz pointed out).
#6-8 are much better, with #6 being perfect. #7 and 8 I'd raise the background just a bit more 1/3 to 2/3 stop should do it...