Define 'casual player'. If you manage to do it in a way that lines up with what anybody else says, let me know. They've been trying to do this on the WoW forum for years, without success. Nobody agrees on what a 'casual' is, or just what splits them up from the 'hardcore'.
Intelligent, thoughtful discussions on the WoW forums (or any forum based on a Blizzard title) are like the existance of life on other planets; it's possible that it exists, but I haven't seen evidence to suggest that it actually does.
A clear definition of Casual Gamer depends upon the discussion at hand and who's using the term. In the most general sense, it's self-explanatory: someone who plays video games casually; they don't pour thousands of hours into a single title and often don't wish to put in large abouts of effort in order to learn/play/win. Pick up, play game, see credits.
Any game that is designed to have no learning curve, as in its mechanics are simplified for - whats known as - the lowest common denominator of gamers, or a game that requires little to no effort on behalf of the player to play/complete the game is generally accepted as a 'Casual Game'. Basically, if you can pick it up and play it without having ever played a video game before and 'win', it's a casual game. The hallmark of a casual game is that it is often a shallow experience that has little depth of gameplay due to the simplification of it's core mechanics. However, that is not to say a 'Casual Game' cannot be played by the 'core' gamers, or vice versa.
Several games that would be considered casual under this general definition, such as Call of Duty: Modern Warfare 2, are not considered 'casual' because of their 'core' gamer following, namely for it's multiplayer aspect. There are always expections to the general rule, and although I word a clear, solid defition to surve as a general set of guidelines, the document would be several thousands words long to ensure I've covered every possible situation.
In short: if a game requires very little to no effort from the player to succeed within the game, the game is considered a casual game.
I'd contend that Civ has always been a great 'casual' game, because of it's turn based nature. It doesn't require twitch reflexes, hours of uninterrupted game time on a tight schedule, or coordination as part of a team. You can play at your own pace, at your own difficulty level, whenever and however you want.
Any game with a save system and multiple difficulty levels falls into that rather loose definition or description. Civilization games are by no means casual; you need to learn the mechanics of the game to actually play it, and then need to learn the more complicated relationships of resources, units, etc. to actually succeed. It requires effort to learn the game, and thus is not considered a casual game.
The basic mechanics are generally pretty simple and not hard to understand except when obscured by poor UI...
They're not hard to understand if you already have a basic understanding; confronted with Civilization II for the first time, a casual gamer would probably walk away rather than learn the finer points of the game. Civilization IV streamlined the understand and, as you rightly pointed out, the game became easier for first time players to understand. It by no means lessened the effort required to play the game, it simply made it easier to understand the complexities already in place.
Altering the game to appeal to Casual Gamers, such as has been mentioned with Civilization V, means lessoning the mechanics in place to require less effort to learn/play/win. This isn't streamlining the UI like Civilization IV to make it easier to understand, this is making the game dumber so that there is simply less to understand.
