Intro
I loved Europa Universalis III (grand strategy), Endless Space (4X) and Galactic Civilizations II (4X). All of those have their own problems but they're generally engaging. Stellaris is not. Some positives first.
Positives
The race-design options are truly impressive. You can pick from a variety of traits, government types, philosophical leaning, et cetera. You can be undying robots, a hive mind or a race that eats the enemy. There are neat options for your starting planet too, like it being post-apocalyptic or pre-apocalyptic. If the rest of the game was half as fun as this it'd be amazing.
The game also features semi-randomised research options which make research feel a bit less stale than Endless Space or GalCiv.
Poor Design
This game is what i call "pause-based". There's probably a proper word for it but it means it's real-time but you can pause at any point and keep playing. Surveys, research, building and the like all take a certain amount of time. Generally you want to play on the highest speed because otherwise it would take forever to get anywhere.
Here's the problem. There's no autopause other than for events. There's no automation option for explorer ships that actually automates them - anomalies/research have to be done by hand. Messages for newly available options are limited. On planets you can build a building for every 5 population. There's no message that tells you when you reach this threshold, and the population of planets isn't listed in your overview thingie. So you constantly have to check this manually. I was playing in a small galaxy and this was already a pain.
Lack Of Fun
Stellaris, like EU, puts some fairly strong limits on expansion. Influence builds extremely slowly, administration takes dedicated buildings, you can't move pops as easily as in many other games, and so forth. Problem is, there isn't a whole lot to do other than exploring and very limited building. No intricate diplomacy and trade, no small skirmishes in neutral territory, no figuring out what unique buildings to construct. Mostly you're just waiting for things to happen.
The one interesting thing is exploration, with a bunch of different events and anomalies and sometimes multiple options. But even during the second game i had already seen most of the events i got. Apparently contact with other factions tends to bog down in stalemates too.
One time a completely new race spawned and right away they crushed my strongest fleet despite it having nearly twice their strength. All the time, resources and research that went into it was for nothing just so this new race could stand a chance i guess? I get that you want some semblance of balance but to do this on the easiest difficulty? I completely lost interest after that.