I’m sure it’s worth your time if you want to jump back in. You can buy a month of gametime and decide if you want to continue after that. As for guilds, it depends on your realm and faction, though I’d say that there are guilds for people like you on the majority of realms.
Honestly, I’m not too sure what you should know before going in. The level squish is probably the first thing you’ll notice, since the max level is now 60 and your level 90 character will be a lower level (40-45 I think?). Models and graphics have also evolved a bit since MoP. The rest you’ll figure out when playing, I’m sure.