Yeah, as the next poster did point out, it's a comeback card, and I've been using it as such (I was at one health left when I pulled it out to just massacre the mage).
Perhaps we're just using "comeback" in a different way: Jaraxxus does nothing about to influence a board that is behind. If you're about to lose on-board, chances are you're going to still lose after Jaraxxus. The exceptions is if it's a relatively clear board and the Heal guarantees you the turn.
Just so I know the reasoning and to see what the issues are with the cards, what do you think I should replace them with, and what's the issues with said cards you listed?
Shield Bearer - Basically just a waste of a space as a card. A 0/4 Taunt doesn't really accomplish anything at all. if you want an early game Taunt, just stick to the Voidwalker. It at least has the upside of having the 1 attack and Demonfire can make it a 3/5.
Frostwolf Grunt - Likewise, this is a 2/2 Taunt that doesn't really do anything either. Most notably, it merely just dies to most 1-drops (since they're usually 2/1s) and basically every 2-drop kills him (since they're almost all 2/3 or 3/2s). Having 2 Health as a Taunt is just pretty bad.
Mana Addict - You're not running enough spells to make her really worth it so she's usually stuck as a 1/3. She's gimmicky and is just taking up a card slot that could be spent on better minions.
Pint Sized summoner - Likewise, she's a gimmicky minion that relies on you untapping with her still alive. And on top of that, you're sacrificing tempo by playing a 2/2 on Turn 2 in order to maybe gain a boost on the following turn (which you can't really guarantee to have in hand). On top of that, you don't even have high quality 4-drops to want to accelerate for turn 3.
Succubus - She causes you to straight lose a card and you get a minion that will most likely just trade with another 2-drop. So she doesn't trade for a card that cost more to cast and costs you a card to even get out. Sure, it's nice if she gets to live the dream and actually attack but getting +1/+1 over a normal 2-drop isn't really worth throwing away a card.
Raging Worgen - He's not necessarily bad but you don't really have much synergy with him. He's really just a 3/3 for 3 in this deck.
Raid Leader - If you really want a buffing minion, Dire Wolf Alpha accomplishes basically the same thing but at 2 mana. You'll rarely get to buff 3-4+ minions and Raid Leader just has awful stats for his mana cost.
Tauren Warrior - Ultimately, he just has bad stats for his mana cost to accomodate his Taunt and Enrage. But, again, most 2-drops are 3/2 so he'll typically trade with him so you have a 3-drop that just dies to your average 2-drop. The Enrage is neat on him if you get it off but it's just really not worth it.
Void Terror - He's just a gimmicky card. You can make him eat a minion that will die to Power Overwhelming and absorb some stats, sure, but then you're dedicating multiple cards to him, essentially, and it can be answered with a single Silence or hard removal effect.
Granted, you're most likely using most of these due to not having access to too many non-basics but these are pretty awful cards. On top of that, your deck doesn't really have any focus. It just has a random assortment of relatively low cost minions but almost all of them are low value ones that have bad stats for their cost. That's basically the general trend here for these cards: They don't influence the board when they come into play, are less efficient than other drops of the same (or even less) cost, and don't even do that much when you actually do get to use them. You're better off just running other minions that at least do something for you. For instance, you can look towards running Novice Engineer / Loot Hoarders for 2-drops to at least get you a card when they're done. Faerie Dragon is a really solid "defensive" 2-drop as well since it has immunity from removal spells as well as giving you a 3/2 body. Shattered Sun Cleric is a great 3 drop since you can buff your earlier drops with her so they get more value. Harvest Golem is basically the defacto best 3-drop since it gives you a 2/3 to trade with those 3/2 2-drops but then gives you a 2/1 body after to trade with another 3/2 (or you use the SSC buff on the 2/3 to make it a 3/4, etc.)
Also, I'd say that Summoning Portal is pretty bad for similar reasons to Pint Sized Summoner: A gimmicky card that has the whole "Needs to be protected, does nothing itself" aspect to it. You're not generating tempo or really influencing the game by playing one. Sense Demons doesn't really do much but I suppose you're running it so you can tutor up your Jaraxxus as needed. I'd say to just cut it and run an actual card you'd be happy to put into play when you draw it.
Edit: You also probably want some mid-game creatures of note. From the basic set, Chillwind Yeti is a really solid addition for a 4-drop since he gives you a 4/5 body that is hard to get rid of.