The thing about an open forum like Gransnet is that it is inclusive by nature: anyone can join in anytime anywhere on the forums. Like every other human endeavour that has been going for a while it has picked up some habits. There are two options for people who are recent joiners: muck in and get used to the habits (all the 'oldies' had to right from the start because the basic acronyms were adopted from Mumsnet before Gransnet had any members; why should now joiner-inners be different from then joiner-inners?), or, secondly, adopt the evolutionary approach that elegran has touched on.
In my experience there are very few acronyms other than the standard mumsnet/gransnet ones used so, really, I think people who are complaining are just enjoying whining about something they're not used to. Which is fine, I hasten to add, but so is a certain level of lack of sympathy from people who have buckled down and got used to the 'culture' in question.