I'm old casual player, thinking to reroll horde warlock. I do have 90 human warlock and panda warrior at Frostmourne almost a year ago but just wanna start the leveling again, on the horde side, just for fun.

Is demo viable for leveling, instance and bg? I have not been playing demo lock since I guest WOTLK so it is a totally new stuff for me. I am always on the affliction side because I'm just too addicted with dots, but it is no longer as strong as before for KB, prolly his role is to put pressures around.

Thinking whether to stick at Frostmourne since it is a high population realm and my golds are all there, or Gundrak..Is there a problem with low population realm because one of my friend says that it is abit hard to sell your stuff..no idea atm since I haven't been into low pop realm before.

Which realm and guild has more casual player?

Well I treat myself as a new player I just wanna learn from fresh and enjoy the game again, probably when i reach 90, can hook up for raid, it times allow
Before it dies.

Frostmourne or Gundrak or Barthilas or Darkspear?
Any opinion?

I'm still undecided which realm yet, I have a friend who is going to start leveling with me on horde side too, see where he want to go. If we decided on frostmourne prolly we can group up for questing/bg/instance
Add my battletag Helven#1977

I'm in Gundrak now, still in Silvermoon city, have not feel the real "dead" yet since this city is dead in every server. No guild invitation spam like FM so far lol.