I'm not a top player by any means, but unless I drastically like one of the new DLC characters, I will likely continue playing Liu Kang since I now have the time to really work with him. I was down on him for quite a while, but the more I looked into it the more I realized that he's probably a solid character that doesn't really have a huge amount of weaknesses, moreso just relatively lacking strengths. But strengths are strengths, and I feel he does a few things very well (This is all in DF; I feel Flame Fist and Dualist have severe weaknesses that prevent LK from being as good as he can be):
+Control of mid range-Liu has a lot of good footsie tools such as low fireball, b2, f2, b1, and Double Dragon Kick that let him force a guess on a person trying to come in. Stopping an onslaught of offensive maneuvers is key to success in MKX, and Liu Kang has a lot of good ways to handle that.
+Meterless damage output-Due to being able to cancel his fireball, Liu Kang gets some really good damage (33+, I believe) meterless off of his most common combo starters. This makes him less reliable on meter to do a lot of damage, which lets him use it for other things that are really useful
+Blockstring offense-Liu Kang, much like his Shaolin cousin, can really run the gamut as far as blockstrings go. F213 fb cancel F213 is uninterruptable if performed without delay, and even a normal dash cancel leaves him + enough to go for b1, which will either break armor or beat a poke, and then go into Double Dragon Kick, which can be meter burned to steal frames again while being uncrouchable. This shaves off 7 or so % and builds meter like no one's business. Add to the many different ways he can go for throw, overhead or low in the sequence, and it's a fairly decent game to play
-Bad armored moves: This one hurts. Backfist, Bike Kick and Double Dragon Kick all have armor, but all have huge issues with hitting characters who are hitting low pokes or doing relatively fast moves. He can at least launch from them with the use of excess meter, which he builds easily, but still not quite as effective. A big blow
-Weak reward on 50/50: Liu Kang has decent ways to setup a strict guess, but his reward off of it is pretty weak compared to most of the other characters. Big damage off the low f4, but b2 and throw (which will essentially function as a second overhead since he forces crouch block most of the time) lead to about 7-12%, which means that he's going to have to land a lot of those to steal a match.
-Highly meter dependent: This one isn't quite as bad, given how much reps of f213 xx fbc will build, but for Liu Kang to safely buy him some time to rebuild his stamina bar as well as get in off a d4 xx cancel or convert off a landed flying kick, Liu needs meter. Between using EN Double Dragon Kick to stay + and in their face from mid range as well as converting off a bicycle kick from f4, Liu will use his meter quite a bit and that can be a problem when it comes time to breaker. Since he builds it really good it's not a crippling weakness, but a weakness nonetheless.
Overall, I think that's where he stands. If I had to boil it down to a tl;dr, it would be that his damage off of the most likely scenarios is not that high and below what the best chars and even the next best set of chars can get. Even then, with his fast footspeed, different options to AA, and relatively simple but effective mid range game, I think Liu can place given a player who's a student of the game and can make the most of his advantageous situations.
