Hi there. I run the type of guild you’ve just described. It was set up early 2007 and, for the majority of that time, I’ve been the guild leader.
We started out just as a levelling/questing guild, but these days we’ve expanded a little 
We now run two raid nights a week - currently concentrating on heroic, working on the last boss on a Wednesday, and an alt run/more casual run on a Friday which is heroic from the first bosses. When the new raid comes out, we’ll be going back to running it in Normal, before progressing into Heroic, eventually. Normally at that point, for some time, we’ll be running both the heroic and the normal raids each week.
Now … that makes us sound like we’re quite organised and like we kinda clear stuff regularly … but we don’t We don’t have any attendance or rank requirements for raids - basically if you reach the gear requirement, you can sign up. We take people, normally, on a first come, first served basis (depending on role - sometimes if we have a lot of healers or tanks, we will run a rota).
We have a few people who sometimes organise a “PvP Friday” - after our Friday raid they’ll go out and have a bit of a laugh in some BG’s or, if they’re feeling really brave, Arenas.
We have some organising “Mythic Monday”, where they have some low key, casual, slow M+ runs to help people learn tactics and get the confidence to go to the higher keys.
We also have a few who are starting to push the 15+ keys in Mythic +, with an active M+ discord channel where people organise groups.
Other than that … we’re a social, casual group of adults (18+), most of whom have families, jobs, real life issues, who just want to relax and play WoW for a while. We understand that real life comes first and that sometimes you just feel the urge to go kill some easy stuff rather than working on progression raid bosses.
We’re made up of altaholics, pet collectors, mount collectors and achievement chasers.
If you want to know more, check out our realm advert: [H]<Dark Wolves> Social, Casual Raiding & Mythic+