Cycling

Total time to design cycling 5 minutes; total time to break it... 4 minutes

Cycling is a broken mechanic; the idea is great, but the implementation is flawed.  There are countless ways to qualify the issue, and some design mistakes are bound to happen from time to time, but ultimately none of the new engines should have made it to release, nor should most of the best individual cycling cards either.

If SOI block was analogous to paper MtG's Urza block (i.e. rampant powercreep that significantly increased the typical speed of a game and significantly reduced the interactivity), and KLD block was analogous to Masques (i.e. mostly reduced power levels overall, with a few instances of much more oppressive cards than intended), then AKH is PQ's Mirrodin, with cycling mirroring affinity in its overwhelming speed, power, and consistency.

These are just a few examples of what a cycling deck can do even without New Perspectives or Drake Haven (Alhammarret's Archive should work just as well as Curator of Mysteries in Standard, though Shefet Monitor has no equal):

Training Grounds using an AKH-only deck (first attempt with this version):

Beginning of turn:

End of turn:

In the most recent black Trial of the Planes event, I ran AKH-only T2 decks for all games and went 12-0 against (mostly) enhanced Legacy decks (in Platinum tier).

In the current FiRF cycle (also in Platinum), a Kiora deck similar to the one depicted has so far managed 3 turn 6 wins with 1 Vehicle replacing Vizier as the primary win condition; in other words, the AI hasn't had a single turn yet in which it would benefit from Enraged on that node.

Sandwurm Convergence doesn't even make the cut in most of my Standard Kiora decks now, because it's weak and painfully slow by comparison.

That's how powerful cycling is in its current form, and again, none of these decks include the very best cycling engines, as I don't have either New Perspectives or Drake Haven.  The mechanic essentially revives an arguably even stronger version of Baral mere weeks after his (well-deserved) nerf, and unlike Legacy loop decks, which are still far too powerful in their own right, this incarnation requires nothing from prior sets to function at maximum efficiency.

@Hibernum_JC Please rethink the broader design philosophy behind looping mechanics and cards, especially in blue; I'm a Johnny at heart (though perhaps more of a Spike/Johnny in practice), so I actually love finding and breaking exactly these kinds of combo decks (especially when they turn out to be competitively viable), but they're not healthy overall when they dominate the metagame and relegate every other archetype to tier 1.5 at best.  Infinite combos have their place (ideally in non-competitive PvE formats), but turns 3-5 of Standard shouldn't be among them.