Creature type themes aren't new, they've been around since Magic. In both Magic and Hearthstone, I love them, and enjoy them, but I think they're bad design that dumbs down the game (wait, what?)
Creature type based decks are way too easy to tune and tweak, and in that way they remove a lot of deckbuilding skill from the game. Creatures are simply "tagged" for whether they will work in your deck and accept bonuses from your creature type enablers or not (in Magic they are sometimes colloquially called lords, in Hearthstone maybe we should call dudes that reward you for playing a creature type warleaders).
As an example, when players entered Managrind in October 2013 the game was nothing but mirrors, which makes for a good abstract example. Demonfire wasn't good at the moment so there was basically no creature types matters at all. There were tons of zoo decks, but one clever guy, Kistafer, cut a couple cards (probably Dark Iron Dwarf) to put Blood Knights in and snatch Squire shields in the mirror and he won. Most players didn't come up with that, or they made their own tweak that wasn't quite as good, like they might have tried Ironbeak Owl or MC tech instead. Every minion was a possible deck tweak. (This was the event where Blood Knight zoo was pretty much invented for the first time in public knowledge, so he wasn't copying anything)
If that same tournament was full of mech mirrors, Kistafer wouldn't be able to cut Piloted Shredder for Blood Knight because Piloted Shredder is a mech and Blood Knight isn't and that will usually outweigh anything a offtype creature brings. The players that think Ironbeak Owl has a little merit will also realize that Owl simply isn't a mech. So when Kistafer plays a mirror against this guy he doesn't get to show that he has a better perception of the game. What few unused mechs might exist will be exhaustively tested for merit easily. So it goes like 50/50.
Murlocs are really cool in Hearthstone because there aren't enough of them. Murloc decks have to run lots of nonMurloc slots because not enough differently named Murlocs exist. So those other slots show off player deckbuilding/tuning skill, and the murlocs are there to provide a unique mechanic, and everyone is happy.
I'm also somewhat placated by designs where all the creatures in a creature type are really weak on their own (murlocs are like this too, actually) but have unusually powerful lords available to make you want to play them. The deck is still really easy to tune, but at least the deck is doing something unique that you do that easily without the creature type based design.
So I'm kind of worried about mechs in the next set. So many of them are neutral so there's some amount of chance they permeate everything. Mechs are pretty much designed to stand on their own in power level (Harvest Golem is a mech!) so the creature type mechanics won't be adding the novelty I referred to in the previous paragraph, just restrictions on tuning for mech decks. I think a Tinkertown Technician that gains one health and draws a spare part if you control any other minion would be better for the game. I don't think having 4/4 Tinkertown Technician in my life will outweigh playing a meta where decks build themselves because so many choices are restricted by mechs-matter.
Players tend to really enjoy creature types matter. Easy deckbuilding is nice sometimes, especially in the short term. It's also flavorful, although mechs are nonliving so they picked like the worst creature type ever imo.
As a potent example of how creature type matters can lead to really canned decks, I have the kithkin story. Virtually all kithkin are only in the Lorwyn set where lots of powerful Kithkin lords are also available for the aggressively costed creatures. It was a pretty powerful deck to build in Standard, where only the most recent two sets are allowed.
There was a legacy tournament , where all the sets were allowed. Kithkin isn't considered strong enough for legacy, and generally it isn't but the point is that one guy entered a very big tournament and made fourth, and didn't modify the standard kithkin deck he had at all even though all sets the game had to offer were legal. He only put 3 Wraths of God in his sideboard, everything else was also standard legal and pretty much Lorwyn. Any other white creature he put in simply wouldn't be a kithkin.
Merfolk and Goblins are two other Legacy decks that don't change a lot between iterations, although they benefit from a large number of creatures printed in various sets to get some more variety, and Merfolk has some interesting choices in the spell department.
Anyway, I'm hoping this new set doesn't have 3 more mechwarpers and a harvest golem.