The latest Hearthstone expansion brought with it some fantastic single player content - Dungeon Runs. I think people would agree generally that it is by far the best single player content the game has ever had, and has far more replay value than anything before it.
The idea is that you start off with a tiny deck of cards standardized for your class. You then fight a series of 8 bosses which start off completely trivial and get harder as you go. If you ever lose a game, the run ends. After every boss you get 3 new cards to add to your deck, chosen from 3 stacks of 3 cards each. Each stack is themed, so you can try to build a deck around a specific mechanic or card if you like. The trick is that there are lots of themes so if you try to build a Jade Shaman deck you might not get offered any Jade cards... or worse, you take Jade cards early and then never get any more and they end up being terrible.
You also get 4 special items as the run progresses. After boss 1 and boss 5 you get a choice of a passive item that buffs you, and after boss 3 and 7 you get an active item that is a card in your deck. All of the items have powerful effects, some moreso than others. The combination of passive and active effects with the variable sets of cards and random boss selection means that the games play out in all kinds of interesting ways and things change each time you do a run.
The biggest problem with previous single player content in Hearthstone was that it got repetitive super fast. Any boss that was a serious challenge would have ludicrous abilities and would destroy you effortlessly, so you simply build a deck that has some possible combination that beats the boss and then play over and over until RNG favoured you. You might be simply restarting until you got 2 Molten Giants in your hand by turn 2 on Lich King - Hunter, or maybe you were trying to land Innervate / AlarmoBot on Chromaggus. In any case these old fights were often tedious to beat because even once you figured it out you just played the optimal deck over and over until you hit just the right combination of cards.
Dungeon Runs don't suffer from this. While there is definitely RNG in terms of what cards and items you get on your runs the games are different each time and there are many ways to succeed. Even if it is frustrating to lose to a particular combination you can try again and build a new deck and face new bosses. The sense of pointless grinding isn't the same at all, and I am enjoying playing Dungeon Runs still despite having beat a full 8 boss run on every class.
One thing I did notice is that my intuition about what cards are good wasn't correct when I started out. I remember the first choice I had between various passive items and one of them was the Potion of Vitality, which doubles your health. Instead I took a Crystal Gem, which gives you 1 extra mana crystal. I liked the idea of having more mana for better tempo far more than I liked a bunch of health. Who wants to be tough when you can just steamroll your opponents?
That was a mistake. In the first few encounters where the bosses are easy that extra mana crystal is huge. It lets you quickly overpower them. However, when you get to the later stages the bosses often have board wipes and have a ton of health so you can't just stick a couple early minions and win - that tempo advantage isn't nearly enough to let you win the game. Since you have 40-50 health in the harder matches the health doubler is giving you a *huge* health increase, which is really important against bosses that try to burn you out with spells or if you try to fatigue opponents out.
If we look at the effect of having one more mana crystal we can compare it to a spell that does that - Wild Growth. This effect is better than Wild Growth because it works on turn 1 and 2 and doesn't cost 2 mana to cast, but we can safely say it is a similar effect that gives about 4 additional mana. Roughly speaking, it is worth a card and 4 mana. The health boost is a much larger effect, when considering cards, because best card to compare it to is Greater Healing Potion, which restores 12 health for 4 mana. That card isn't all that exciting though, so perhaps we should only give the Vitality Potion credit for being as good as 12 mana and 4 cards. 12 mana and 4 cards is a *lot* better than 4 mana and 1 card!
The Vitality Potion is even better when you can usefully leverage that extra health. Classes with weapons can safely chop away at enemy minions without worry when they have Vitality Potion, and if you manage to get a full heal card it is even better.
The decks you end up with at the end vary quite a bit in power. Certain classes have a much higher peak than others, particularly any class that has the Jade mechanic "Summon a Jade Golem. The first Jade Golem is 1/1, and each subsequent Jade Golem is +1/+1 bigger." The treasure that doubles Battlecries means that the first Jade card you play summons a 1/1 and a 2/2, the second one summons a 3/3 and a 4/4, etc. With a bunch of Jade cards and doubled battlecries you can steamroll anything. Classes that don't have Jade as an option can't highroll the same way, although there are two treasures called the Hilt of Quel'Delar and the Blade of Quel'Delar, and if you get both they turn into a weapon that basically instantly wins you the game. You can only get it for the 8th fight of the run and you have to pick the first half of Quel'Delar before knowing if you will even get offered the second half, but when it works you just win.
I have found it super interesting that people have wildly differing ideas of the power level of various items. Some are obvious, such as the Captured Flag which gives your minions +1/+1. It is excellent, one of the best for every class and strategy. However, there is one in particular, the Cloak of Invisibility, that seems to have some serious disagreement on its strength. It makes all of your minions permanently have stealth so your opponent cannot trade into your minions and they cannot target them. You can dictate all trades and protect your minions with special effects, but you do have the real problem that you lose access to Taunt. If you need to protect your face from enemy minions you are screwed because your dorks are all hiding.
I love the Cloak of Invisibility. One of the huge benefits of it is hard to see, but is really important. Some enemy decks have a truckload of removal in them and it can end up clogging their hand badly when they can't target your minions. The first time I beat Vustrasz he played 2 little minions and then just sat there beating on me with them doing nothing, eventually discarding a card each turn. Clearly he drew all targetted removal spells and other minions he couldn't play and my invisible minions happily beat him to death. Things don't always go to that extreme of course, but oftentimes bosses will end games with 5 cards in hand that they haven't played and it is clear that the Cloak of Invisibility was responsible for stranding all that removal in their hand.
On a somewhat lighter note I must say I really like the feel of the Dungeon Runs too. The monsters are varied and there are some rare fights with fun rewards. You actually have to use a lot of different strategies depending on what you are facing and they really stuck the feel of delving into an old school dungeon. Giant Rats as your first opponent, and dragons, demons, and The Darkness at the end (with a Lava Filled Chamber in the middle, naturally) hit all the right notes.
If you haven't played Hearthstone in awhile this is a plenty good reason to pick it up again. The solo content is free, and you get a bunch of free stuff for logging in during the expansion.