There are some points why I consider KSP unfinished:
The 'career mode' only provides some random side quests combined with some very weird tech progression concept ("lick a flagpole to develop a new engine") and arbitrary constraints (Part count? Really?). There is no meaningful use of the "time" resource, management of parallel missions is pretty much impossible without mods, the "mission control" building is just a tavern for quest givers and the less said about the "administration" building the better... The whole mode just feels like a hastily added collection of half-baked ideas that had to be included so that the 'released' product gives at least lip service to the game's tag line: "KSP is a game where the players create and manage their own space program. Build spacecraft, fly them, and try to help the Kerbals to fulfill their ultimate mission of conquering space."
Part balance is all over the place:
Fuel tanks all have identical dry/wet mass ratios and all pros and cons that need weighing is "the 'physics' engine prefers small part counts". The same is true for batteries.
Manned pods in contrast have pretty random mass/kerbal values without any secondary property to consider. Lander cans and aircraft parts are just more efficient in every way as capsules, description texts hinting at differences regarding survivability of re-entry or vacuum conditions are just cosmetic.
Probe cores: You use the Stayputnik if you have nothing better, then the next one and in the end the lightest one. Without research, you just use the lightest one. There is no reason whatsoever to use things like the 1.25m or 2.5m probe cores or the second of the hexagonal/octagonal cores.
Graphics: Many objects still have graphics that are okay for an early alpha but somehow still survive to the day. Furthermore, there is no consistent art style which adds to the "unfinished" look.
The "work in progress" label that is tagged to everything that receives any amount of criticism from the community. Yes, the label is not always given by developers but also by zealous members of the community, but reading "well, it's not yet finished" in pretty much every discussion will nonetheless add to the "unfinished" look as well.
To the "just pick a version that you like and stay with it" argument: Mods. Yes, it is possible to get the old versions of mods, but most modders will put their own continuous development only in the current version (and I do not begrudge this, they are not paid for it after all) and especially if this development contains bugfixes and performance/usability updates or simply interesting new features you are pretty much left with Hobson's choice: Do not use these improvements or update the game (and all the other mods).
[Emphasize mine] That sounds a lot like KSP...