K&C introduces a new mechanic, Recruit, which summons a minion from your deck and drops them into play. Perfect, right? So efficient! But most Recruit cards are conditional (e.g. only bringing creatures of a certain cost), so you'll have to build your decks carefully. As mentioned, each class gets its own legendary-quality weapon, most of which have powerful or lingering effects that make them useful for far more than whacking enemies.
There are also two new card types. Spellstones are spells that increase in power after certain conditions are met keyed to your class (suffer damage as a Warlock, etc), and once powered up, can swing the game in your favor. In keeping with the expansion's Dungeons and Dragons-esque theme, the other new cards are Unidentified Items, which randomly adopt one of four upgrades every game.
The single-player mode, Dungeon Run, will excite players who have mourned the absence of solo content after Blizzard switched to releasing only full expansions this year. Previously, they interspersed larger 135-card releases with smaller 45-content drops called Adventures that centered around player-versus-computer experiences. To make up for it, Blizzard promised to add free solo content to the new full expansions, beginning with the previous release Knights of the Frozen Throne. Whether that makes up for how much more expensive it is to keep up with the game without the far-cheaper Adventures is up to each player.
Kobolds and Catacombs is the last of 2017's three planned expansions. When the first big content release drops next year (likely in April), it will phase out 2016's cards from the 'Standard' mode of play, which only keeps the current and previous year's cards.