This is very useful, and to call it disappointing is rude to the creator of the tool. Why would you say something so negative just because it's not perfect? This is the problem with Hacker News these days - people can't wait to say something nasty.
I have edited my post to hopefully make it less "rude", that was certainly not my intention. I do want to point out, and I think I am not alone in this, that in 2012, lack of IPv6 support (at least at the basic level of parsing IPv6 addresses and showing AAAA records / reverse lookups) in a network tools suite doesn't make it "not perfect", it makes it "not useful".
Au contraire, if anything, consistent rudeness risks getting you hellbanned.
I don't think it's unreasonable or rude to state that one is disappointed that a tool doesn't have a feature. Especially an important feature such as IPv6! Given that the disappointment is that ViewDNS.info bills itself as "your one source for DNS tools", it doesn't entirely live up to it's own billing.
Why is having a valid criticism or critique considered rude?
If I was the creator I would welcome such feedback.
If anything I would say that's one of the positive aspects of Hacker News.
Pointing out the flaws in a service is better than not posting at all. The creator can then use these criticisms and make the service better.
EDIT: for friendliness.