You've heard the news already: Standard and Modern have been changed again.
Let's look at Modern first.
The two-year experiment with Golgari Grave-Troll is over. The powerful dredge enabler finally tipped the scales as more and more players adapted to either playing the powerful deck, or packing hate specifically to target it. It's been called "the best in Modern" recently, and while the format continues to surprise (Skred Red, seriously!) it's clear Dredge is not the type of Magic Wizards want to see that successful.
Gitaxian Probe is a bit different to get the boot, but it's in line with the ban on Preordain: The ability to efficiently massage and dig in a deck only lets the good decks get better. Like Mental Misstep, the cost of just one Phyrexian mana means it's effectively a "free" spell. Without the limitation of mana at work, getting a free dredge trigger, fueling things like Become Immense, or even making Death's Shadow a thing put Probe into a class of cards where your deck is worse without it.
While Emrakul met defeat in the finals of Pro Tour Eldritch Moon, the power of the Edrazi leapfrogged expectations thanks to Aetherworks Marvel and the finely-tuned Black-Green Delirium lists.
Neither deck was getting worse with Aether Revolt, but that's not the the real crime: Players found "getting Emrakul'd" thoroughly unpleasant. Thanks to the rules oddites of looping nested Emrakul, the Promised End extra turns being taken against either other, Emrakul's time in the spotlight was brought to an end.
Similarly, one of the keynote cards of Kaladesh is gone as well. Smuggler's Copter was played in a variety of decks, from the aforementioned Delirium-style decks that want to discard things to a wide range of aggro. As the centerpiece of the Vehicles archetypes it put a few players into the Pro Tour Kaladesh Top 8 and become a staple of Top 8s. It's popularity, as a function of Wizards pushing the first design of a new card type, became its undoing.
Reflector Mage is likely the biggest surprise, but it's not unwelcome. Since Oath of the Gatewatch released, the Mage locked opponents out and became fast friends with Eldrazi Displacer. Anyone playing white and blue threw it into decks, and the function of stymieing opponents on an iterative basis means even innocuous cards like Panharmonicon rose to become Standard standouts.
What Comes Next?
Wizards also announced that five weeks after each Pro Tour the Banned and Restricted list will be updated again. While they emphasized that this doesn't necessarily mean adjustments for formats will occur more often, doubling the number of opportunities for it happen leads to the opposite conclusion. Fun-to-play is a subjective point of view, but the decisions Wizards makes is ostensibly driven by data.
Is attendance down? Are distributor sales down? Do Pro Tours need more marketing potential? It's not clear from the outside why Wizards is focusing on so many changes that affect Standard, but with their complete emphasis and marketing drive tied to the format means we can expect to see more adjustments in the future.
Let's hope they're solid.