I think the biggest problem with culture is that you only need one planet to win with it.  Culture needs to be weaker at large distances than it currently is.

Maybe a two tiered solution.  Overwhelming culture flips normally (ie if they colonize a planet in the heart of your empire you should have a decent chance of being able to flip it without issue) but flipping planets at the current strength requires something like a declaration of war that comes with a large diplomacy hit.

You could call it a cold war or a culture war or something like that.

Another possible solution is that once espionage is in current strength flipping requires espionage actions against the planet you're trying to take.

Edit: It would also be nice if my culture didn't take planets from my allies.

The thing we're all missing here is, if you're taking them over culturally, and they are happy with your culture, then of course, they aren't going to respond militarily, in fact, they'll tighten the relationship between your two people, until, at its logical conclusion, the two become one.

I reject the seeming primary assumption of this game that the entirety must revolve around combat.

I keep reading something along these lines on the forum; "if they're happy with you, they stay happy with you?  That's crazy!"

Personally, I find the super-snarky, dripping-with-violence "benevolent" civilizations "crazy"... aren't they supposed to be the "good guys"?

Is this "american politics"-style "good guys", do as we say or we'll drone you?

Can we take a brief moment, and look at "lawful good" in Dungeons and Dragons, and then review "Benevolent" in the dictionary?

Pretty sure this "friendly-type warfaring race" doesn't fit the bill.

As stated above, I like that you can win by culture.  For me, in an ideal game, all possible victories would be as close to equally achievable as possible.  I find that culture is a far easier way to win than any other method.  That is my problem.  In fact, I have trouble enjoying the game because I keep accidentally winning cultural victories, including over my allies.  And I think Yeller puts it perfectly, the people of a planet want revolution, its government certainly does not, especially if they're already at war with me.  Keep in mind, I never WANT a cultural victory. In fact, I intentionally stifle my culture most of the time so i don't spontaneously take over the galaxy.  

Also, using the influence line of perks in the benevolent ideology tree at all is basically instant cultural victory.  If the +100% influence to home world doesn't make things one sided then the third perk where everything in your influence instantly swaps (including entire minor races) definitely will.  The last time I used that perk the galaxy went from being roughly a third mine (out of three players) to being completely mine in maybe 5 turns.  No fun.

Maybe in multi player this is more balanced because humans are smart enough to generate counter-culture and correctly use ideology, but as the AI are currently it's no longer any fun to play against them because I know I'll end up with another cultural victory.