Demo tanking is significantly easier as horde than as alliance. Why? Because running from ally FR to horde FR is a downhill slope, ans speaking as someone who's been there on both factions, your demo leaps send you a MUCH further distance when you're running the right direction
Demo FCing is an extremely interesting side hobby, although it is much, much more feasible at 29 than at 20. Glyphed hand of gul'dan gives you an aoe snare that, as opposed to a 20 demo lock's only snare that costs 80 demo nrgy out of your typical 200 starting, costs nothing and actively GENERATES demo fury, giving you more jumps(the bread and butter of demonology). Hand of gul'dan should almost NEVER be used for damage; ALWAYS think of it in terms of snare utility number one.
You also have an incredibly underutilized defense against rogues and druids! Hellfire. Combined with your level 27 passive health regen, you can make your entire flag run under the effects of hellfire, and knock stealthies out before they can open on you four times out of five. If you see a stealthie go into stealth, you can leap into their projected location and be hellfiring them out of stealth before you land.
NEVER waste your demo fury on offense if you're carrying the flag! You should be constantly dotting corruption to gain demo fury, and using your fury for only two things: demo leaping, and staying in demo form whenever a physical damage user is upon you or if a stealthie is expected. The armor bonus is great; I think mine was 38% or something in demo form. Keep a VERY close eye on your demo form cooldown. If you have to go into demo form for a leap, don't leave demo form until cooldown is off; you do NOT want to get caught by a melee when your armored form is on cooldown!
And although I often feel I am alone is this, haste beats the shit out of versatility for a FCing set for DEMO LOCKS ONLY. Coming from someone who started FCing using vers and fell in love with haste after giving it a shot, haste has some unsurmountable advantges. You will generate MUCH more demonic fury by getting more corruption/hand of gul'dan ticks, giving you a significantly higher uptime on meta form, which translates directly to less damage taken for a longer period of time. Your hellfire will tick MUCH faster, giving your anti-feral ability that much more use. And, my personal favorite, your fears are THAT much quicker. If a stealthie gets the open on you, demo leaping directly into a haste-fueled fear can keep you alive.
And of course, possibly the most important part of lock FCing is this: DO NOT GIMP YOUR TEAM! If there's a top-tier FC out there like thot or someone else, let them take the flag, or pick it up and run it directly do them. No matter how good you are at demo FCing, there are only a few situations in which you'll be able to out-FC a GOOD druid or other FCing spec. There are plenty of opportunities to FC in games in which there simply AREN'T better options or your current FC sucks the nuts, but being cognizant of your surroundings and your teammates' strengths is important
that's all I remember off the top of my head. I mostly play SC2 right now but I went through a phase in which I loved demo tanking
edit: best of luck mayne!! Hope you have fun giving it a shot. Any fun you can squeeze into demo right now is worth it since the spec will be completely unrecognizable once legion pops out
Love seeing demo warlocks!