I've been lurking on this board for a while, but seeing this question I finally had to register & post!
I think 2nd grade really matters. I have one son who was an amazingly enthusiastic learner K & 1st in a great public school system with an extensive gifted differentiation program. But we moved, and he had a wasted year in the new public school 2nd grade classroom. Because we had made no progress in advocating for him over the year, for 3rd we switched to a private school which was more rigorous and more flexible. (I still feel guilty that we solved our problem with $$ and did nothing to help those coming after us who may not be fortunate enough to have that choice. I ask my husband, what if Ralph Nader had only installed seat belts in his own cars, and not pushed for them in all cars? But that's another post...) I think this son stil suffers from his lost year. He has never quite regained his zest for learning. (He has DYS-qualifying scores on WJIII and Explore, but I'm still procrastinating on the portfolio thing. If I get it together I am hoping they can help me figure out how to help him get his spark back.)
On the other hand, his younger brother started K at the private school. The kindergarten teacher challenged him in math & reading, and helped him catch up to the other kids with his fine motor skills. He had an awesome year and literally skipped around the school and the house. We debated a full grade skip of 1st, but decided to keep him in 1st and accelerate him in math only to 2nd. He was miserable the first two weeks of school. Other than math he was bored stiff, and all the 1st grade work played to his weakness (fine motor skills). Every night saw fights and tears. Thankfully, by 10 days into school, the 2nd grade teacher approached US telling us that we'd made the wrong decision and he needed to be in her classroom full time. She worked it out with the school director and the 1st grade teacher, and voila, full grade acceleration.
Now he is finishing his 3rd grade year, still skips around home & school, and loves a challenge. He took the Explore test this Feb, and got DYS-qualifying score for 3rd. He even did better than his 5th grade brother in one or two subjects!
So, we worry all the time about the future of the son who was not challenged in 2nd grade, and are delighted with the progress of the son who WAS challenged in 2nd grade. I am sure there are plenty of other factors at work here, but we have certainly concluded that 2nd grade does matter!