Honestly, I think the biggest issue with Stellaris is something that hasn't really been addressed by the devs. Right now, there's zero replayability. I think part of this stems from the fact that the game is an awkward mashup between a grand strategy and a 4X, but there's really no reason to play twice because once you've played to the end once, you've pretty much seen it all outside some flavour events. Putting aside the fact that right now the mid-endgame is a boring repetitive slog, something that is important in most 4Xs (and strategy games in general) is making the different sides you play as actually feel different from each other, and Stellaris does not accomplish that. Each race is only different by cosmetics, because you can customize ethics and gov type, and the ethics and gov types only provide small percentage bonuses one way or another. Playing as an individual materalist science government and a communal, spiritualist oligarchy will give you pretty much the same experience, outside of the silly flavour events that always happen for spiritualist. There's a severe lack of unique mechanics for gov types/ethics. Yeah, you're more able to have slaves as a communal species, and more able to purge as a xenophobic species, but that won't really change how you play your game. Everything still has the same boring ass endgoal, which is just to paint the map in your colour by slogging through hundreds of boring identical wars with all the federations that form. And even if the endgame events weren't totally broken, you can see all three in one playthrough, giving no reason to play again to try and find more. Even the planets you can settle makes no real difference. It's just either a boon or an inconvenience depending on how lucky you are with RNG, until you get to the midgame and the point becomes moot.
I'm sure unique mechanics for gov types and ethics will come in the inevitable rush of DLC, but they should be ashamed of the state they released the game in. By the midgame I had to force myself to keep playing because I wanted to see the endgame stuff, and once I realized how boring and broken they were, I quit. Tried again as a different race with different ethics, gov, etc, but it was just more of the same. That IGN was spot on, and I personally would've given it a lower score than 6.5, because only half of one game was any fun.
It sucks, I've been playing Paradox games since HOI2, and this is the first one that I've actively disliked. I love 4Xs and I love Paradox, but I just don't find Stellaris to be any fun past the early game, and I have no desire to play it again. We'll see what they do with DLCs, but I'm pretty disappointed.