1. What specific features of diplomacy do you traditionally like the most?
Even though it's a given, having some way to quickly balance the two sides of the deal. So for example, in GC2, the bc needed to make things equal would be picked out. In Civ, asking 'What will it take to make this deal work?'
I like long-term co-operation with other civs, as opposed to just taking everything by force. And if the terms of co-operation change through the ages, that's something I can live with.
I think that more than anything, I remember those times when Civs stayed in character and didn't try to stab me in the back.
2. Looking back, how many turns do your favorite games last?
Hundreds. Close to a thousand for GC2, and in Civ 4 for example I would typically play Marathon from the Ancient age. With slower tech progression. And timed victory turned off.
3. Consider all The 4X strategy games that you have ever played. How do you define what is a good strategy game or a bad one?
I guess I would say that the thing which makes a 4X bad is not having the right level of challenge. The AI opponents or the environment should not be easily beaten or tamed. I remember thinking that in Alpha Centauri or Civ II, for example, you had a lot of tools to radically alter the environment, which basically meant that at one point or another you would win simply because you were godlike even though you hadn't actually passed any of the victory conditions yet.
Having a challenge doesn't necessarily make a 4X good though, it just means that the player will keep coming back for more.
What makes a 4X good? Um, well I think with a lot of games allowing you to customise (for example GalCiv or Civ, being able to customise your Civ) I think it's about the player feeling like the timeline of their struggle tells a story of victory (or defeat) and that their choices and actions helped to shape that story.
It's that lucky and memorable discovery of a high-PQ planet or Precursor Library in GalCiv which helped things along. It's that goodie hut which yielded a much-needed technology or Settler. It's deciding not to trust a particular AI player and being justified when they try to take over the world - and you're ready for them even if the other players aren't.
You are a storyteller, and a general, and a diplomat, and sometimes a brutal despot who had slaves whipped mercilessly to get results. You've been there at every step along the way, from that lowly first Archer unit right up until the thermonuclear holocaust which destroyed most of the world. Oops. There's continuity, you know, your aptly-named capital city standing the test of time. Unless someone razes it to the ground, of course, in which case it continues to be ashes until you get a settler in there to rebuild it.
What is good about a 4X is expanding your mind to consider new possibilities, new strategies and different ways of playing. So for example, in Civ 5 I liked that each civ has different advantages and ways of playing. In GalCiv 2, the default races gained unique improvements techs, and abilities.
It's not about cranking out generic victory X in the least turns possible, or achieving a million point score. Well, it may be to some, but not to me.
It's about exploiting your knowledge of relations between the other civs to make yourself appear to be the trustworthy trading partner.
It's about sneaking your spies in to sabotage production (without going complete overkill like in Civ 4 and having so many spies that your opponents can't produce anything). Or there's intrigue, which was introduced in Civ 5: Brave New World, and allows you to warn players of impending sneak attacks.
I think in a nutshell, 4X games are good because there isn't one cookie-cutter way to win, things happen and you may have to adjust your plans on the fly.
Hope this helps!