No overload sucks. Think about what it really lets you do.
You play a 4 mana card on turn 3. Awesome right? Well the problem is those cards are not as good as the cards released since the start of the game. So you are really playing a 3.2 mana card on turn 3. Meh.
Worse, on turn 4 you can only again play a 3 mana card, which is a big ptoblem, because your opponent is playing an even more valuable version of a card that is better than yours to start with.
Not to mention you just died anyway because of all the aggro decks.
The value from overload simply isn't there. The cards carrying it are outdated and crap.
Exactly. Making an existing 3-drop in to a class card (!) as is, except as a 2-drop (overload 1, so it's really a 3-mana again).
This is what Shaman gets. A neutral card made in to their class card. It's a joke.
And innervate vs overload? Haha. That's hilarious, innervate creates game-winning combos. Turn 1 Emperor, Turn 2 Boom, Savage Roar + Savage Roar + FoN, etc. Overload generally responds to board and then screws you over the next turn (lava burst an Emperor, now they get to play their hand at a reduced cost and you get to pass your next turn, wonderful).
Overload cards are extremely outdated, and even with Lava Shock, you still sometimes can't optimally use the overload (either because there aren't enough mana crystals or you simply have no reason to overload atm but need to use the Lava Shock).
Edit: Let's face it, the most popular classes are the ones that have and can create the most broken outcomes from cheap combos (I'm looking at you, Mad Scientist and the synergy with Mana Wyrm / Unstable Portal into some game winning card and those 2/4 fire throwers and the 3/2 mage cards that reduce spells by 1, and the then 2 mana draw two cards... see synergy. Mage is like a whole class that cards all works wonderfully together and they have stuff like Unstable Portal that costs 2-mana and wins games). That's what every class needs to be competitive, and Shaman is definitely lacking it, outside of the 3/2 windfury mech when your opponent lacks a way to remove it.
Simply put, for a class to be popular, it needs a broken combo, that generates an incredible amount of tempo or advantage. It needs it early on, and it needs a TON of synergy between cards (see Mage decks). If you don't want to give the class such a thing, then it needs another way to deal with the onslaught. Because when people play against Shaman, there's really nothing that they have to worry about, except maybe having a 2-damage removal. It's not like playing against other classes like Warlock where you really need to hunt for BGH or Hunter where you need your early game or you lose.