Hearthpwn

People don't use her, because she doesn't generate the needed value you are expecting, she's way too slow and demands a situational deck to fit in. + If you want to use the above mentioned combo, you have to wait a turn to do so, because she generates an imp at the end of the turn, when you cannot interact with anything. + You have to destroy a minion of your own, in order to get that health boost.

Let's talk about taunts for a little bit. Generally speaking the health total of a taunt transfers to your own hero, because it soaks up the damage, which you could have taken. However that's not the case with spells, which normally bypass minions.  When you use your hero power (in the lock class) or any other class specific card, that deals damage to you, you are getting pretty weak (= vulnerable) and you need something to sustain you. Something reliable. Something, that works very well in every situation. On the one hand taunts slow down pirates, but it's not like these type of decks don't have any answer at all. Very often, when I played my second rate bruiser for 3 mana, my opponent played heroic strike for 2 with a little ping of his weapon, which all combined destroyed my taunt. It soaked up the damage - true, but the minions on his side of the board remain untouched + he has unused mana to spend and you are still in trouble.

Let's give Reno for an example or, because some people hate him, antique healbot. The amount of health they restore to your hero is huge. They can save you in every situation - when there's a big minion threatening lethal or when your opponent has burst from hand. Taunts like Tar Lurker can work very well against the first case, but not against the second one. And (I'm sorry i have to mention this) if you want to make a competitive deck, you should include cards with good quality. Let's compare Imp Master with the combo, you've suggested, against an aggro deck (against pirates). When you play her, she becomes 1/4 + 1/1 imp at the end of the turn. Turn four begins and Garrosh puts his artillery on the board, he of course ignores your board and goes straight to the face. You use your 0 mana Pact, you gain 5 health and then what? Or you use that dinosaur, like you've mentioned. Don't think that you will always find 3 health + taunt on that minion. Even if you do, Reaper + a small pirate and bam, the taunt is gone. In other words - your suggested combo will not work at all, or maybe in come cases, when your opponent has bad draws (or bad cards).

Tl;dr: I'm not saying, that we need neutral cards will full heal like reno, but at least good healing cards. especially for classes like lock, mage, rogue etc, which don't have any.

Quote from Vernula >>

The thing about Standard is that it's constantly going to change. You just had Reno Jackson, prior to that you had Antique Healbot. You'll probably not see strong heals for a while now, but the taunts they're giving you will be "heals" in the way that taunts are.

Warlock may become a class that has to really question if Lifetap is worth it later on in the coming year.

 Let's talk about taunts for a little bit. Generally speaking the health total of a taunt transfers to your own hero, because it soaks up the damage, which you could have taken. However that's not the case with spells, which normally bypass minions.  

Vernula, I know, that you are a fan of the warlock class as much as I am and especially of the control type.
I'm not a whiner, that reno is leaving. I read some forums that Antique Healbot was being included way too much in some decks, which resolved in limiting the space for other cards. And frankly this is true. Same thing is with reno, kinda. 
Let me however disagree with you about one point. Having a good heal option in standard, will always be a plus for the decks w/o it. A taunt can't be that flexible as it's heal equivalent.  As explained in the quote, taunts soak up the damage, that is being dealt by minions. But you can't always be sure, that this will be enough for you to survive. In the last (pirate) meta we needed more ways to undo the damage, the taunts really did a great work at blocking them. But what IF you don't draw the taunts at the right time. Imagine how you are on low health and you draw your big body minion. Would you always feel safe, when you put it on the board, even if the board is empty? Would you feel, that after playing it, the game will go in your favor or that you are making a comeback? Because every class has a reach, every class can do well against a single minion. The heal options can undo the damage at any moment - in mid game or in the late, when you are struggling.

Imagine what it would be, if handlocks didn't have antique healbot. Yes, they would still use Molten Giant (I'm acknowledging handlock before the molten nerf), but would it be enough against the hunter's spells and hero power? I will never complain about taunts - they are essential in this game, but they cannot replace the healing. And it's not only the warlock hero power, you also have other cards like Abyssal or Hell Fire, that deal damage to you. Earthen Ring Farseer is good in the early game, but not in the late. Cult Apothecary also heals you for a bunch, but 1) it's situational; 2) if the opponent has a big board, it's more likely that they will use the numbers to finish you (Bloodlust, Savage Roar,the zoolock's Darkshire Councilman benefits with the board presence) 3) even if you combine him with Shadowflame, it's 9 mana, which means that you can do this only in the late game.  A good healing option would be Jinyu Waterspeaker, make it 5 mana, neutral and voila - not too oppressive and not too much space limiting.