We originally struggled with a few points that weren't clearly worded, but those were all addressed in forums by the Modiphius folks, and the Quickstart was updated to clarify those issues.
In our test with the revised QS rules, I think the main thing was just getting our heads around the ways that the three resources (Momentum, Fortune, and Doom) interlock; when it was better to spend Fortune or add Doom to the pool, what effects to choose from the suggested Momentum and Doom examples,
etc. My advice is to walk very slowly through the first round of an action scene, just to get the interactions down, and then "rewind" and start the scene again at full speed once folks are comfortable with the basic mechanics.
The only other thing that's tricky is learning to read regular six-sided dice as Combat Dice (1 = 1 pt.; 2 = 2 pts.; 3-4 = nothing; 5-6 = 1 pt. plus an Effect). Again, we found that this is hard for about two rolls, and then you get it, and from then on it's trivial.
I've boiled the Quickstart Rules (plus addenda mentioned in various fora by the Modiphius designers) into a 7-page summary PDF, which you can check out at
https://drive.google.com/open?id=0B0ck1une_YLJdmYwbmh1VlFQQUU. It doesn't replace the Quickstart rules; it wouldn't be as good for learning the concepts, but I've found it to be a nice fast reference for use in play. Comments welcomed.