Guild needs:
1 A good/motivated officer team.
They can and will carry the raid with organisation, having the correct tactic which will work for your guild. Being able to sort out disputes/arguments properly so your players dont hate the environment they raid in and actually want to show up.
2. Players who can follow instructions while playing their class to some competent level.
You dont have to have rank 1 dps to kill bosses. Eventually gear will let you kill bosses but if you have 0 clue what you are doing then no one will want to actually play with you if you dont put in the effort
3. Roster of 24/5 ish people who turn up regularly.
Kicking people for the smallest transgression or not maintaining your roster through recruiting will cause mythic guilds to fail. Imo im a firm believer that taking chances on players who dont have the gear/exp is a good way to breed decent mythic raiders. We all started somwhere just need a little faith.
4. Guild of people who dont want to kill each other or hate each other.
This kills guilds every single day. Just having a toxic raid environment isnt fun for anyone.
5. The willpower to survive 100+ wipes. Imo this is most important. If you cant wipe for hours on end during progression what is the point.
Wiping is part of the game. People without patience will just hamper your teams performance.
There is more but too long to write it all down xD. Hope this helped