Unlike some of the other people, I do not need to tell you what you "should" do. It's going to be your decision, either way. However, at level 22 your needs at this time will not be the same when you are over level 40, so you can plan your point allocation wisely, and change them at the appropriate time to save money.
I noticed that right away, and fortunately, I did not have to re-specialize more than three times (I'm almost 64). Demonology is probably the slowest to get you some noticeable results, which is why Affliction or Destruction may be good to start, though the top two tiers of Demonology can help you early-on.
Talents are just some extra power to what you already possess, so as stated, there is no inherent need to build toward anything.
Demonology is mostly about your minions, not yourself; your benefits will be minor, so you have to love your minions to commit to this specialization. Use any gear or talent that will boost your defense, stamina and intellect; these attributes make the minions stronger, along side yourself. Eventually, the higher your minion's stamina and intellect, the more bonus spell damage you and they will gain. At my level, my pets are hard to kill (even in PVP), and I usually die before they come close. My felhunter is probably the most contended, and at level 55, was close to unbeatable (unassisted) that was close in level.
Felguard really is not as fancy as it seems. His defense is much lower than the voidwalker, but his melee is what makes him last short fights-- he is not good for anything drawn-out. I use my voidwalker and felfunter in shifts, because they both contribute a lot, when soloing. My felhunter is my favourite for PVP, because his abilities seem to be the most pervasive.
A good Demonology warlock is going to make use of all of their minions (I never use the succubus, though), not just one of them. Realistically, just one is not going to be very helpful, and some people do not like the felguard in instances, because he can bring too many on a party. At my level, I still use my imp-- he gives me the most bonus damage, and makes me less tastey to enemies, so I can whomp them, while other party members are getting beat.
Demonic Sacrifice only sounds cool. Unless you have another specialization, and have developed it more than Demonology, then there is really very few instances when losing a pet is a good idea.
Demonology warlocks get a lot of power form their minions, and visa versa. When one of them is gone, you are stripped of a lot of bonuses that are quite useful. I like to get bonus-damage gear, which counters the need to buff Affliction or Demonology any more. For example, my Drain Life already does twice the intended amount of healing/damage, and my curses seem to do around 25% more damage or more, not counting criticals.
That brings me to another point. I think that bonus damage is more important than critical damage. Not because you will get more damage, but because you will always deal damage in a specific interval. Criticals are the chance to do deal extra damage. Unless you have over 20% critical chance, then you will not notice much of a difference. Demonic Tactics will increase your critical chances, but even after mastering it, I notice that my bonus damage is more reliable. However, the felhunter seems to have a high critical rating already, so you can still find some practical use from critical damge bonuses.
Of couse, that is just how I have developed a Demonology warlock, and that is how I have learned to deal more damage. That may not apply to other specializations.