I agree with the others on filters. I have the 16-35L, the 24-70L , the 70-200L and the 100-400L lenses and the 1.4 extender. I love them all. I just got back from Central Asia and didn't take the wide and regretted it (was worried about carry-on weight). But my pictures were great. I agree on getting 2.8 or better for low light. I almost never use a flash.

I've had great luck adding photo filters in Photoshop, so I really don't use the color filters any more (except warming filters in Egypt--I know I'll want yellower colors in the sand and pyramids and putting them on is easier than photoshop)