First, the simple expansion of the acronym "NPE." It means Negative Play Experience.
Second, of course this quick explanation is my own opinion, and -- like porn -- NPE is to at least some extent something that's tough to define, but you knows it when you sees it.
Third, what an NPE is not : a conclusion reached as the result of losing a game. Or two. Or even ten. The possible causes for loss of game are many, and varied, and -- and this is important -- the proper assessment of NPE is almost totally divorced from whether or not the assessor is losing games. That is, someone who can judge an NPE well does it just as validly by winning with it, as by losing with it ... and even by simply watching gameplay as an observer.
So what is an NPE?
In general, an NPE is something that, largely independent of skill or luck, significantly reduces the enjoyment of games, for one or both players, involving a particular concept, component, combination, or process. As follows from this, NPEs are actually pretty rare. The combination of "it isn't just because you lose to it," with "it needs to be something you can't avoid with reasonably equivalent skill to your opponent" means that it just doesn't happen often.
The biggest recent examples? Phantoms. Super-turrets with Boost.
Background: I love Phantoms. I particularly love Echo. But as a Phantom lover, I noticed something: unless someone brought a counter or mirror to my list, or unless I completely screwed up, I won when I was playing Phantoms. There was nothing a non-counter list could do but hope for me to make a catastrophic mistake.
So, even though I won with Phantoms, it quickly pinged my NPE radar.
What's an example of something that is not an NPE out of the gate? Large-based turret ships. There are a bazillion ways to beat them, ranging from list selection through skill to reasonable luck.
Now add in actionless defenses: C-3PO, Ysanne Isarde. These are things that specifically mitigate or eliminate a major weakness of typical Large ships: namely Agility. But, you know what? Still not terrible. Powerful, yes, but still beatable by the simple expedient of getting "guns on target." Still not an NPE.
But now add in the last piece, the mitigator for even the ability to beat Large ships with "guns on target" ... Large-ship Boost.
So now a Large ship can (a) withstand a lot of damage and never lose firepower, (b) increase its defenses so that when it does take damage, it takes less, meaning the high HPs go even further, and © range out away from multiple guns, so that it's disadvantage in firepower disappears and it's fighting effectively one-on-one with a smaller, more fragile, slower, arc-dependent ship. There is nearly nothing that an equivalently skilled player can do to prevent these cumulative effects from maing a game all but unwinnable.
And that is how "not an NPE" becomes an NPE.
