In my gaming experience, I've come to realise that I generally encounter 2 main types of NPE, both of which have been talked about in length here:
1: Poor deck management: This to me is when a player builds not a whole deck (regardless of the game being played) but opts to put in one or two cards- such as the angel combo pointed out earlier. This is not gaming. When the game asks you to build a deck of 40 or 60 cards, building a deck of 2-4 cards and filling the rest with other cards designed to stall play long enough to play said "game winners" takes away the entire experience of gaming, which starts at deck building, not with the first hand of the game.
2: A-hole gamers: There's a few different types I've encountered in my days, and some are worse than others. There are the players who are so arrogant that they would rather talk down than talk to their opponents. Being better at a game means you should be trying to help those who are worse, not demeaning them. Also, any player who intentionally tries to draw out games and take away the experience itself. I'm not referring to stall decks, which is a viable and occasionally fun deck style, I'm talking about the players themselves, who dispute every decision made in the game, just to draw it out more. I'm a firm believer in being social and talking while playing because gaming is a social event, but when some people make it their personal mission to interupt the flow of the game to argue or stall, it kinda ruins the experience IMO.