SLX is Shimano’s new all mountain/hardcore trail groupset, which sits below the Saint in price and in intended use.
Although the SLX initially appears to be the Hone with a new name, there are a couple of innovations. There’s two Hollowtech II cranks: a standard triple with 44/32/22T rings, where the 32 is a composite steel construction like the XT, and a double with 36/22T chainrings, steel pedal thread insert and plastic bash guard. To complete the double Shimano has produced a dedicated front derailleur designed with a radius specifically for two rings. It says it offers extra tyre, chain and stay clearance and will be available in top-swing and down-swing versions. There’s no specific shifter but apparently the three-speed one works fine.
The rest of the groupset is pretty standard fare, with a Shadow rear derailleur in medium (GS) and long (SGS) cage options, Rapidfire Plus shifters with 2-Way Release, Servo Wave brake levers with tool-free reach adjustment and a 20mm through-axle front hub.

Apparently the new 15mm thru axle (called E-thru) developed by Shimano and Fox will be available as an option on all Fox forks currently not using the existing 20mm axle; namely Fox’s 100, 120 and 140mm Float and Vanilla forks.
Shimano claims the new standard “saves a lot of weight compared to 20mm, while keeping almost the same rigidity,” but has yet to provide detailed weights or stiffness figures. It also claims “15mm allows not only a weight reduction on the hub, but also on the fork lowers.” So how light or stiff the whole system is and whether it’s worth the hassle we won’t really know until we get forks and wheels to test at the press camps.
A crumb of good news is that the new standard is a completely open format so other fork, hub and axle makers can participate.