Many games are praised because of how time flies as you follow your nose from one objective to the next, and though this layering of goals and rewards is a meaningful and valuable part of an experience, I tend to knock it down if it ultimately leaves me with that "wasted my time" vibe. I'm sure the line is different for everyone, but if I can reasonably predict the results of gameplay systems interacting but the game keeps me going because of how it layers goals and rewards, then I want to limit the credit I grant it. If another game has less nuanced gameplay interactions, but it keeps feeding me new (i.e., unpredictable) input, I don't walk away with the "wasted my time" vibe and want to give it more credit.
But can it be broken down so simply? Exceptions to the rule pop in my head too easily. For example, I can reasonably predict the results of a night of Street Fighter, Tribes, or Subspace but I still give them high marks. Maybe the exception to the rule is competitive multiplayer; that player unpredictability is more interesting than single-player unpredictability to me. I don't appreciate random events driven by numbers nearly as much.