The pendulum swings, but make no mistake, its resting state is not the center.
Every set needs to introduce new mechanics to entice players, even if it offers a new gameplay experience by remixing existing mechanics; that's [one of] the biggest reason[s] the core sets were so unpopular for so long. But how many new mechanics does a set need? 1? 3? 5+?
Kaladesh's success demonstrates 3 is one ideal.
Remember New World Order? The concept was shared with the world in late 2011. Time Spiral had taught R&D that putting too many mechanics in a set made the game too complex for new players to approach, so they cut back in Lorwyn and realized board complexity was a deal-breaker too. NWO promised to lower the barrier to entry for future sets, while preserving a deep gameplay experience for established players. The plan was to keep the game's complexity at rare and uncommon, and out of common, so that each game will see a few complex cards that keep it unique and interesting, but not so many that players are bogged down by walls of text, myriad options, and on-board tricks.
It worked well enough that they felt comfortable joking about doubling down with New New World Order. (Many of the ideas in NNWO were valid at a lower threshold). In fact, as the blocks rolled on, even the principles of NWO were bent more and more, with blocks introducing 11 or 12 new named mechanics, and texty abilities crowding even common text boxes. Don't get me wrong, we were still better off than before NWO, and the core principles behind it continued to be leveraged and to improve Magic's accessibility, but as the proverbial pendulum swung between slow blocks and fast blocks, between gold blocks and artifact blocks, between two-sided conflicts and five-sided ones, it also trended toward more complexity again, over time. Which it always will.
But here's Kaladesh, with three new mechanics and no returning mechanics (save the evergreen ones, obviously). Fabricate is deadly simple (but so functional). Vehicles have some complexity but not too much: We'll get used to them just like we did equipment, but for now we're still sorting out how to value them, and getting used to their gameplay implications (like crewing them on defense, or letting you tap your creatures freely). Energy is straight-up alien. The rules are very easy to understand, but it's taking players a while to fully understand the mix-and-match nature of energy—we're not used to mana as an accumulating resource.
And it's good. The pendulum has swung back toward lower complexity again, and the game's still great.
So where's the threshold between too much and not enough? M10-M15 addressed the biggest complaints established players had with core sets—that they were boring because it's all old news—and had a great deal more success, but still couldn't compete with expansion sets. Visiting a world is a big deal, and WotC markets each new plane a lot harder than core sets and supplemental products; that's part of it. But it's also very true that one of the things Magic players love about Magic, is how it reinvents itself. We want / need / and expect to see something new.
A block that introduces something truly novel, like double-faced cards or energy, can get away with less named mechanics—new or old*, as Kaladesh shows. More traditional blocks can distinguish themselves based on the combination of more traditional mechanics, and the unique gameplay environment they support, and that will usually require more named mechanics to keep the players satisfied.
(* While it's important to think of past keywords as being reusable, it's better to focus on the driving force behind that: That each set should use what it needs to be novel and fun, and nothing more. Sometimes, that means using old keywords, sometimes old ones, and sometimes just fewer.)
Kaladesh delivers on the same high-strategy-high-variance experience Magic always has, while offering something completely new to explore. Anything more would just muddy the message and dilute the experience. Aether Revolt will add a new spin on that, but not before we're ready for it.