As a rule, I avoid free-to-play games. The only one I can think of that I play is Crusader Kings II, and I bought it years before it was free. There are a number of things that are common in them that I dislike:
- They often have "pay to win" schemes, particularly if they are multiplayer.
- They are often very repetitive if you don't fork over money
- The mechanics are often very light. Civ, on the other hand, is anything but that.
- There are often gambling elements (e.g. lootboxes). I personally don't believe these should be legal, and support the steps Belgium has taken to outlaw them
So what would I find acceptable as free-to-play games?
- Actual free games, e.g. Freeciv or Battle of Wesnoth. Obviously, not a great business model for a company, but I have no objections to them.
- The only paid content being cosmetic items, and those having stated prices (not gambling/lootboxes). AFAIK, games like League of Legends follow this model.
- The base game being free (and substantial), with paid DLC packs. This is more feasible for an established game like Crusader Kings II. But it has to make it not look like the core is intentionally sparse, e.g. The Sims 4, or Railway Empire (which aren't free, but are commonly criticized as being shells designed to sell DLC).
As a hypothetical example, if Microsoft had made Age of Empires II free - but only with random maps - and then charged for the various scenarios (Genghis Khan, Joan of Arc, etc.) and the eventual Conquerors expansion, that would have been fair. I suppose this could also be looked at as having a "super demo" with extras available for those who are willing to pay for it.
Generally, however, so many free games having one or more of the flaws in the first list, that I ignore them all as a rule of thumb to save me the trouble of separating the wheat from the chaff. I'd rather pay upfront and have an more reliably enjoyable experience.