I started the transition to being vegetarian in 2004, and completely gave up meat by January 2005. For the most part I am very tolerant of others, I pretty much never mention vegetarianism at all. But if someone brings it up to me and wants to initiate a conversation, I'm all over it.
I believe the reason a lot of new veggies are less tolerant is because they have just realized a great injustice is going on and are in the throws of changing their lifestyles. They have have a revelation of sorts and expect everybody around them to be having the same ideas. I know when I first had the idea that being a veggie was the way to go, I didn't understand why the people I was telling about it didn't seem to agree with me. Over time though, you learn that some people haven't thought about it yet, but will in the future, and that others just don't care. That was a bit ranty of me, but that's the general idea i get from the intolerance of new veggies. It's harmless, they're just so excited that they don't understand why others arent joining them in their new lifestyles.