Kangaskhan is awesome. It is the very definition of a self sufficient Pokemon, and is able to function as possibly the most effective glue in the tier, fitting into pretty much any team that isn't stall. Despite this however, I agree with the above posts in that it should move down to Mid S.
One of Kangaskhan's main problems, I find, is that sometimes it just misses out on those kills or 2hkos. Especially when using its coverage move in either Drain Punch or Earthquake, it just lacks that bit more power needed to kill. This can be compared to Pokemon like Electivire in RU, though not on such an extreme scale. Earthquake just doesn't do enough damage, even to things like Bastiodon, which are 4x weak to it. This means that Drain Punch is insufficient even more of the time, often unable to get those crucial kills needed. When compared to other offensive normal types, such as Tauros, Zangoose, Swellow or Braviary, its damage output can be disheartening. As wiitle addressed above, it is very much a one trick pony. Any player with experience in NU can predict what you are going to do fairly easily while you have Kangaskhan out. Almost all physically defensive Pokemon shut it down completely, while forcing it to take recoil damage from Double-Edge (which is generally superior to Return). The recoil taken is often very high, due to defensive Pokemon usually having high hp (Alomomola, Lickilicky, Audino etc.). After taking the hit, they can go ahead and just recover off the damage you just did to them, or simply attack you or status you. This leaves Kangaskhan neutered for the entirety of the match, and can simply cause it to be extra baggage.
The fact remains that it is still a brilliant Pokemon, able to abuse its ability Scrappy to hit everything with its fairly powerful Double-Edge. Kangaskhan has good stats all around and, stat-wise, is a jack of all trades. It has good attack, good speed and surprisingly impressive bulk, able to take hits much better than would be expected for a primarily offensive Pokemon. The main reason it is such a good Pokemon is its unique dual priority, which enables it to become possibly the best revenge killer in NU currently. Kangaskhan can get many kills a match by coming in on a weakened Pokemon, faking it out, then sucker punching as it tries to finish you off, or simply killing with Double-Edge/Earthquake if you know you're faster.
Unfortunately for Kangaskhan, the metagame has shaped around it, and many walls have risen to be more prominent, which puts Kangaskhan a a disadvantage, as it can no longer abuse its priority to such great effect. Mandibuzz, the most recent and final drop before X/Y, is a massive problem for Kangaskhan. It can easily play around Kanga, roosting off any damage done by Double-Edge and Toxic stalling or simply recoil stalling until Kanga dies. The rise of play styles like bulky offense, and the gradual rise of stall, have also no done any favours to it, as they once again, limit the effectiveness of Kangaskhan's all important priority, which is a massive drawback to it.
Despite how negative this post has been, Kangaskhan is still a superb Pokemon with a great niche in the current metagame, and is a solid Mid S Rank mon imo.