I did some research of my own instead of waiting for another answer. I played 4 games with my Level 5 Mage (leveling her up to level 8 in the process). In none of the games did I manage to use the whole deck, but I wrote down all the cards that I drew throughout all 4 games.
First, to confirm Resorath's answer, none of the Level 2, 4, and 6 cards of the Mage ever showed up in the deck, but the original 5 cards that the Mage starts with all showed up in at least 3 games out of 4. So I can conclude that none of the later Mage cards are included into the basic deck as you level up.
Second, exactly ten other cards showed up in 2 to 4 games each out of 4, while no other cards ever showed up. So, only 15 different cards ever showed up for the Mage, some twice, making up the deck of 30. That means, it's not random, and the cards are set in for each basic deck. Most likely, they are tailored to each class. (For the curious, for Mage they are: Murloc Raider, Bloodfen Raptor, Novice Engineer, River Crocolisk, Raid Leader, Wolfrider, Oasis Snapjaw, Sen'jin Shieldmasta, Nightblade, and Boulderfist Ogre, plus the original 5 Mage cards)
Both of those conclusions could be very good coincidences, but, eyeballing the probabilities of these coincidences, I firmly believe that those conclusions are correct.
So, I did not experiment with all 9 classes, but I found my answer: The basic decks were pre-made by the creators of the game, and they do not change as you level up. I guess the results for the other 8 can be found the same way.
No reason to concede before you draw, you might get a lucky break, but people should concede more often.
Just play a few turns and see how it's going.
If someone has you at 1 health with a full board and you have no minions, you wouldn't feel obliged to play it out, right?
If your draw is awful and they have great board presence after a turn or two, maybe no need to play out every excruciating turn there either.
There is a middle ground between conceding instantly and holding out hope until the very last moment of every game.
Some games swing back and forth, sure, but if you're trying to win as many games as possible in a limited amount of time, the key is to develop a sense of which games are worth playing for wins, and which games are best ended as soon as possible.
Best Answer
I deal with stealthed minions in one of 3 ways.
Area of effect
Flamestrike, holy nova, fan of knives, doomsayer, etc. can all damage stealth minions, however, the damage they do is not always enough. Mass silence can also help.
Random target spells
Spells that have a random target, such as deadly shot for the hunter deal have a chance to deal with a stealth minion quite nicely. The less minions the enemy has on the board the better (usually).
Taunts
Taunts will force the stealth creature to attack it instead of something you want to stay alive. The opposing player could quite easily get rid of your taunt, though, and it doesn't work against minions, who are not made to attack, such as blood imps.