In the few times I've played MKX online, it's actually been remarkably good. Some flakiness is bound to happen, but my attitudes about the game aren't defined by online anyway.
Good stuff
+Variation system is cool in concept, even if there's a lot of room for improvement.
+Remarkably well balanced, considering the size of the cast.
+Reasonable progression of MK9's good points (delayed wakeup, modest buffer, et cetera) makes it pretty easy to pick up, with enough technical stuff in FBRCs and Variations for tech junkies to sink their teeth into. This would be a pretty good place to start for someone unfamiliar with the genre (and doesn't like Street Fighter).
+The new characters are decent, and have unique styles--rather than being Jarek-tier reskins.
+The patching has been handled well, at least in comparison to past outings.
+Match intros actually being interesting is a blessing.
+Brutalities are awesome. Some are too particular, but it's well-integrated regardless.
+Solid selection of characters with reasonably distinct playstyles. It's great to see fan-favorites like Tremor return with a fresh coat of paint (fingers crossed for Drahmin or Mavado).
+I expected the interactive stuff to be terrible, but I'm impressed with how they turned out. Having them cost stamina and being blockable is lovely. They're pretty diverse, and can turn the tables in interesting ways. All in all they encourage stages to mean something, which I like.
Bad stuff
-Variations suffer from the expected problem of one generally being 'best'. Some characters only have one decent Variation, leaving them feeling gutted.
-Some characters got a lot more love in the Variation concepts and variety. Also inevitable, but it doesn't make Jacqui or Reptile's any more exciting.
-"Press X to Purchase Goro"
-Stamina feels like it hurts more than it helps. Dashes never seem to be anything other than 'awesome' or 'awful', and making backdashes cost stamina is iffy. Why should any backdash be crappy (like Kenshi's) when it already costs half your stamina? It doesn't add up.
-Speaking of Kenshi, how about that zoning, eh? They really turned down the jets, which is a bummer. Fewer effective playstyles leads to less diverse gameplay.
-Improvident move choices. So many normals, strings and special moves with no apparent purpose beyond window dressing.
-I think Breakers costing stamina (or at least all of it) is too extreme. It's not like the backdashes are so good that people would do them immediately, surely? This only ever seems to hurt characters with already lousy defense. This goes double for Block Breakers, the refugees of the drawing board.
-The handling of the frame data is an embarrassment. NRS needs to take a page from Virtua Fighter.
-Story mode was weak.
Overall, I like it pretty much the same as MK9, maybe a smidgen more. There are problems with the frame data, movesets, and Variations, but their potential is equally important in my eyes. MKX is fine, but it's not that far from being great if NRS plays their cards right.