@Clemens
To me a good turn-based game is NOT based on skill or learning the mechanics. "Skill" in that respect would only mean that you know how to beat the numbers. It's not about being fast or having good reflexes like in first person shooters. There is no skill in games like D:OS. To be able to react with what you have to a challenge ahead of you isn't based on skill but on basic intelligence.
As I said in my post before, I think that the game should offer a lot more randomness. A game like D:OS which is based mostly on number games and simple causal combat mechanism (actio->reacio) is somehow "dead" to me. You beat every enemy by the same patterns or different set of patterns based on their resistencies and stuff.
I think a good game with some "challenge" here should mean that you have to die in hard fights. Maybe two times, maybe ten times, maybe you make it in the first attempt. Since many stuff is based on random principle you can never know how it plays out in the end. That's the basic principle of many board games as well, at least the entertaining ones. They combine tactics and strategy with luck. A game which is only based on one of them is either too chaotic or too boring and expectable...