I like to compare different expansions to each other to examine the impact of the meta... I wonder if Journey to Un'Goro will have a lasting impact on the meta like Whispers of the Old Gods or if most of the legendaries will simply go forgotten like those from The Grand Tournament (now wild).
Let's see what Un'Goro brought to Hearthstone:
This set introduced a new tribe of elementals. There is a lack of decks using the synergies that elementals have expect in certain mage and shaman decks.
In my opinion, Un'Goro wasn't a flop by any means. I do strongly believe the quests were a bad idea and for 2 good reasons:
1. By playing the quest, you are forcing your deck into a single, solitary system, which is both predictable and (in most cases) rather boring. ANd the opponent knows instantly what you are likely to be playing, and to a large extent, the final outcome. This is the whole concept's biggest downfall (and a problem with Hearthstone overall, in fact). These days, you can pretty much have a good idea of whether you will win a matchup from the first 2-4 rounds. (unless someone makes some bad misplays, which doesn't happen so often up here in the higher ranks).
2. The quests were horribly balanced. It's almost like the designers had some random ideas for quests and thought: Hey! Let's go with that! Not to mention the fact that most of the quests can even be easily countered and nullified, effectively ruining your win strategy before you even get it going. You are basically waving a flag to the opponent saying: Here's how I'm going to win this game. You have X number of turns to stop it happening or prepare... oh, wait... you counterspelled my quest?! WTF?
This is why the Deathknights - again, in my opinion - are much more a step in the right direction. They still suffer from the problem of being badly imbalanced in comparison to each other, with Warlock and Shaman getting the best and most OP ones as usual. (Priest is going to be screwed once Raza rotates out...)
Be thankful quests failed, for the exact reason scorpyon stated in his point #1. The quests are entirely "I'm going to do X and you either stop me or you lose", which is terrible gameplay as it is entirely unresponsive. Hearthstone is the weakest when the game is each player trying to do something with really no regard as to what the other player is doing except removing obvious threats. All face decks are like this, but other decks like taunt warrior are the same.
That being said, the expansion actually was pretty solid if you look past quests. The meta before Un'Goro was horrid and Pirate Warrior was poised to run rampant with the rotation of Standard, but Un'Goro it had enough to slow the meta down some. In any expansion, only 25% of the cards are actually viable, and Un'Goro was no different really. Yeah, it had some untapped promise, but I'm personally thankful the quests didn't pan out, or else the game would get really stale.
The only real failure of an expansion was TGT. Had TGT came first before Naxx/GvG/BRM, it actually could have been quite good at the time, but card quality was painfully weak, and was obvious it was weak before it was even released.
In a grand scheme of hearthstone Un'goro was not the most impactful expansion and yet Un'goro at a time was best thing ever. It didn't have any ridicolous cards, but after the hardships of MSoG, un'goro was like getting 37 blowjobs at a same time. MSoG was pretty much Ben Brode's way of saying "Fuck you" to hearthstone community. And than they released Journey and we forgave. Journey will always have a special place in my heart just because it came after the hell hole known as a era of MSoG. Fuck MSoG. And fuck anyone who dares disagree with me. I'm right you are wrong! :P