Other space has been claimed back by ditching the loop recorder section altogether, and the loop section has been tightened by removing the LED display along with two of the FX control bays. Perhaps most disappointing is the demise of the filter knob on the mixer strips -- such a small, but valued component. You can achieve the same functionality via shift+gain , but only for the main track decks, and it's a bit of a cludge as the rotary is incremental, making smooth fades more tricky.
Spin the device around, and there are some key differences around the back, such as no audio or MIDI inputs like before, but you do get a switchable gain control for booth and main, which is new to the S2. The front is almost untouched, bar the mic volume control being swapped out for an mic 'engage' button.
The biggest boon to the user is the overall size. Most likely this is what will sell it to you if you're not switched off by the lower knob count. The S2 is much more bag-friendly, a welcome development for some no doubt, as the S4 isn't friends with your average rucksack, no problem for junior however.
Software
Needless to say, the software integration here is tight, making it feel more like an integrated device, rather than a disparate software/hardware 'best effort' like it can with some generic controllers.
Of course, the Kontrol S2 can be used as a general MIDI controller, but it's fair to say it fully shines when used with Traktor Pro 2, a copy of which is included in the price whether you run a Mac or PC. As such, you're really shelling out for a complete DJ system, rather than an input device, and it is worth accounting for that when you weigh up the over all price.
Needless to say, the software integration here is tight, making it feel more like an integrated device, rather than a disparate software/hardware 'best effort' like it can with some generic controllers. With the S2 connected, launch Traktor, and it will automatically open in a device friendly pre-configured mode. You can of course alter this configuration, but most likely – for beginners and those new to Traktor – you'll want to hang out with this setup for the time being.

Native Instruments' decision to allow the S2 to only work with two track decks might jar with some people, but in practice it make some sense. Of the many configurations available in Traktor, the two decks and two sample decks will be of the most use to the most people, and the S2, being custom built for this, arguably gives it the edge over the S4, albeit just for this configuration. No doubt some people won't be feeling the same, but Native Instruments likely isn't going after you with this device if that is the case.
The competition
The controller market is busy right now, and all the main (and not-so-main) manufacturers want a cut of the action. This market space is still maturing, as vinyl users make the cross-over, and new DJs jump straight in at the digital ground floor. To that end, the main competitors for the S2 right now include the Pioneer Ergo V, Novation's Twitch, Vestax VCI and the Stanton SCS.4DJ, amongst others, all of which have their own particular strengths.
Price-wise, the S2 is pitched towards the top end of the list, with only the Vestax fetching more than the S2's
$669 asking price. You might argue that you're getting Traktor Pro 2 into the bargain, but have you tried using Serato Itch without a controller attached? Exactly. Clearly in the S2's favour is its integration, offering a complete experience that others in this range will struggle to compete with, but it's a differentiating factor they have pushed to the limit with the smaller form factor.
At the beginning of the review we asked if this was a refinement on the previous device, or something aimed at beginners, and the price does have an important part to play in that. In this case it feels like Native Instruments is hedging its bets a little bit and gunning for both the new and those looking for serious kit with a smaller price. It will be interesting to see which of those two groups will prove the keenest adopters.
Wrap-up
Above everything else, the Kontrol S2 does what it's meant to do, and it does it very well. It's fun to use, and it comes with a great heritage. On those terms it's pretty hard not to see this as a probable success. The DJ controller audience are a keen and fickle mob, and if there is even one thing not to their liking, you can be sure they will vocalize it. As such there will be some who malign the omission of certain features, or how there is nothing really new, but over all Native Instruments clearly knows its own software, its previous successes, and the current state of the market – and when you put all that together, the S2 is the result.