Game play is either calculation or guessing. Neither of which are decisions.
Calculation is not really a decision: you calculate correctly and succeed, or you calculate incorrectly and fail, or you don't to calculate - or recognize when you can't, because the information is not available to you - and you guess.
Guessing is also not a decision: you guess with the odds, or against them which is foolish.
Calculation is made difficult by time constraints, limited mental capacity, and false beliefs. Time constraints may be explicitly enforced, such as in Chess or Scrabble, or implicitly enforced by social necessity: your friends will leave if you don't make a move already. Mental capacity limits how much information you can juggle in your head at any one time. False beliefs are those which skew your ability to calculate or guess properly, such as believing in false patterns or that something will happen because you want it to, very badly.
Calculating odds is not guessing. Calculation is the opposite of guessing. Anything you can't calculate you guess, and since you have no information, no guess is better than any other. Deciding whether something really is a guess or can be calculated, is also a calculation. More specifically, in deciding when a pattern is a pattern, and when it's just random noise.
Determining what your opponent will do is often guessing. Game Theory has a lot to say about it, often on the assumption that your opponent is rational and intelligent. But, in essence, your opponent's moves are just as much odds calculations and guesswork as any roll of multiple dice with constraints can be. Which means that even classic abstracts with perfect information - other than your opponent's future moves - are luck, on a certain level. Your opponent may not find the right move, or he might, by accident, when he guesses a play that is beyond his calculation abilities. Over the course of a long game, the odds of his repeatedly finding the right play by accident become slim to non-existent.
In some games, theme, role play, and humor also play a role. Setting this aside, where are the decisions in a game, if they not in the calculation and not in the guessing? Is decision-making an illusion?