I was having a discussion last night with my friend on the decks in this game and drawing comparison to MTG.
Paladin = white weeny deck mostly. Pumping out little 1/1 with some protection to chip away at the opponent. Then land a few big buffs to pop a weeny to something monstrous when the opponent can't counter it.
I rank this one of the better decks to play with once it gets going for all experience levels.
Mage = blue/red control damage deck. Control in this game is mainly minion denying or spell countering through secrets. Mage has the only actual counter spell in the game. Mage is all about pumping your spell dps up as best as possible and countering anything the opponent plays
One of the best decks out there to play from noobs to the experienced players.
Hunter = Green/red rush deck. Some DD dps and lots of little rush creatures with occasional big crush creatures. One of the best decks out there to play from noobs to experienced players
Warlock = Black sacrifice speed deck. Some countering, some DD dps, and big effects at the cost of the warlock. Sometimes it pays off and sometimes it doesn't. Due to randomness of the deck and specialization of combos required, it's not an easy deck to play with. For noobs one of the worst, and even for experienced players not all that great
Cleric = White/Black deck. It has protection/healing from white, and the ability to go DPS sacrifice of black with a little bit of the white weeny too although not as crazy as the paladin deck. It's an out live your opponent deck. Doesn't have much in the way of counters. It's harder for noobs to play but can do well enough with experienced players
Shaman = Blue/green/red semi creature dps heal counter deck. It basically tries to do too much with the cards available. Like warlock it also has a lot of randomness to it which can really help or hurt you. Not exactly the best for noobs to play and not great for experienced
Druid White/Green/Red healing/creature/dd rush deck. Similar to Shaman but more focus on healing and protection and weeny creature rush. Less on DD and randomness. Also has the worst card in the game with naturalize. It can be very fast for a rush deck like a hunter if not faster. But it also tries to do too much and one can get screwed over by trying to rely on too many little combos. It's not a very good noob deck, but better than shaman/warlock. It can be very powerful in the hands of an experienced player. When it crushes it crushes.
Rogue = Blue/black creature and creature counter deck. Some randomness to this deck with some small sacrifice, but nothing like the warlock deck. It has some dps potential, but not really any spike dps. It has some good countering potential though against minion cards. It's an okay noob deck and a decent experienced player deck.
So here how I would rank the decks on a noob friendly scale:
1. Hunter
2. Mage
3. Paladin
4. Cleric
5. Rogue
6. Druid
7. Shaman
8. Warlock
Here is how I would rank them for overall:
1. Mage
2. Hunter
3. Paladin
4. Druid
5. Cleric
6. Rogue
7. Shaman
8. Warlock
Although anyone can technically win with any deck, especially early on, because more cards in a deck are usually neutral cards. Decks are 30 cards with most low level decks only having about 10ish class based cards. Higher level decks have 15ish class based cards or so. So while a warlock deck I think is weaker at level 1 than a level 1 mage deck, a warlock deck can still beat a mage deck due to both mostly relying on neutral class cards.
How I've built and seen decks built:
1) Build for late game. The game isn't quick enough that it's rare someone is going to win by round 7 or earlier. Most games if not all will have both opponents reach 8 mana. Which means having your deck full of low casting cost cards is only going to hurt you in the end game as you run out of cards in 1 round and risk having your opponent countering everything you do in a single round with 1 or 2 cards. So the majority of your cards should be 3+ casting cost.
2) Look for combos if you have the cards. Some decks lend themselves really well to certain combos. Mage does well with spell damage combo cards. Hunter does well with beast minion combo cards. If you don't have the combos cards then go with cards that split threats or eliminate threats when played. Some have already been mentioned, but here are some of the great neutral cards everyone has: All the Summon extra minion Battle cry cards. You force your opponent to have to deal with multiple threats from 1 card. Which forces your opponent to opponent to use 2 actions to deal with 1 action from yourself. Sprinkle in taunters if you don't have combos that rely on them. Look for the bigger taunters with more health over the smaller ones.
3) Build your deck to gain an action advantage over the opponent. You want your opponent having to spend 2 or more actions to counter every one of your actions. You want your actions to counter multiple actions on your opponent. For example, spells like flamestrike, and consecrate wipe out multiple enemy minions with 1 spell. Save your single target minion wipers for the later big game minions that are harder to take down with 1 action otherwise.
4) Always have ways to get more cards. I've only ran out of cards from my deck in this game once and I still won even taking damage from not drawing for 2 rounds. If you run out of cards, chances are the opponent is about to die. Don't be afraid to cycle cards fast.
With those main points in mind, it is easy to see why I ranked the decks as I did. Those decks that are more efficient in actions usage, faster cycling, and build for late game tend to win more consistently in Hearthstone.