Flow and Spelunky

Quite a bit has been written about the flow mental state, and how games can allow players to achieve it, but there is still a lot of misunderstanding about what exactly it is and how it's different from a game's pacing. What Mihály Csíkszentmihályi stresses in his original book -- and what many people writing on the subject seem to ignore -- is that flow can not be forced; The activity must be intrinsically rewarding. In other words, a game must be fun before it can allow a player to achieve flow. Fun and Flow are not interchangeable terms. 

Derek Yu's Spelunky is an excellent example of a video game that is flow-enabling. Let's take a look at how it achieves each of the 9 components of Flow.
1. Clear goals (expectations and rules are discernible and goals are attainable and align appropriately with one's skill set and abilities). Moreover, the challenge level and skill level should both be high.
There are three main types of goals in Spelunky: Exploration, Collection, and Progression.
Exploration refers not just to the physical space, but also the game dynamics and aesthetics. There are hidden secrets, new ways to kill enemies, avoid traps, and so on. Learning how to play the game is a form of exploration. The interplay between enemies, the player, environment, and traps makes the learning process incredibly lengthy. A player may quickly learn that throwing a bomb into a giant spider's web will kill it safely, but what if you have no bombs and the spider is already jumping around? If that makes the goal of killing the giant spider too difficult, the player is able to modify their goal on the fly to one matching their skill level - perhaps fleeing successfully will pose enough of a challenge. BecauseSpelunky never forces the player to do anything except walk through an exit at the end of the level, the goals contained within that level are created and completed or discarded constantly by the player.
Collection goals encompass treasure, power-ups, damsels in distress, and the killing of enemies. These are tracked and displayed at the end of each level and death. Overall totals are also shown to the player. Once again, the player is not forced to collect anything to progress, but the game provides the data necessary for players to make personal goals out of it.
Progression, or the completion of the game, is the only discrete goal the game provides. New environment tilesets and enemies, a final boss, etc. draw the player forward to an ultimate conclusion. However, this goal is equal to if not subservient to the other goal categories. After the game has been beaten once it offers replayability as long as the player is willing to continue striving for more treasure, a quicker run-through, more saved damsals, or maybe even just more creative deaths.
What's important to note about this flow component is that the high difficulty and steep learning curve of Spelunky are actually good for enabling flow. If it's too easy, if anyone can do it, the goals become meaningless, are achieved too easily, or never set by the player in the first place. 
2. Concentrating and focusing, a high degree of concentration on a limited field of attention (a person engaged in the activity will have the opportunity to focus and to delve deeply into it).
A high degree of concentration is obviously needed to survive in Spelunky. The game is unforgiving of mistakes, and it is always clear why the player died. The field of attention is limited to the game screen. Indeed, almost all action off the visible screen is paused, and the player will never be killed by something they cannot see.
3. A loss of the feeling of self-consciousness, the merging of action and awareness.
Like almost all games, this comes with expertise. A good player will not even think about avoiding a trap or enemy, but do it successfully every time.
4. Distorted sense of time, one's subjective experience of time is altered.
Like component 3, this is a mental effect the player experiences, and not something that can be designed for. Or can it?Spelunky allows the player to quickly restart, and even has a suicide hotkey. There is no real penalty except for a resetting of personally appointed goals. This often results in play-throughs of varying lengths from a few seconds to several minutes. In my personal experience, I would often play many brief sessions punctuated by lengthier "successful" runs. Over time the smaller aborts seem to blend together, and the "real" longer attempts seem to be the only ones that matter. But in reality the aborted attempts added up to more time than the games where I progressed through multiple environments. I would sometimes spend over an hour building up to a good run that lasted maybe 9 minutes, and looking at the clock I wonder how the time passed when I was playing for such small increments. The same effect can be observed in multiplayer FPS games such as TF2 or Counter-Strike.
5. Direct and immediate feedback (successes and failures in the course of the activity are apparent, so that behavior can be adjusted as needed).
I don't have anything to add to this. Feedback is a standard component of any game and Spelunky excels at it for reasons I assume are obvious.
6. Balance between ability level and challenge (the activity is neither too easy nor too difficult).
This is traditionally viewed as the holy grail of flow in games that designers reach for, but as you can see it is just a single component. The ability of players to set their own goals dynamically (see earlier giant spider example) is the perfect solution to the problem of Challenge vs. Boredom. The player has the means to straddle the balance, and it is the player who knows where this balance exists for them. No computer algorithm can provide that same level of flexibility or intuition. Challenge level can also be modified through power-ups and items bought at the store. 
7. A sense of personal control over the situation or activity.
Already covered but to recap: Player recognizes every death is fair and due to lack of skill or setting an unattainable goal, there are no explicit goals except those the player sets moment to moment
8. The activity is intrinsically rewarding, so there is an effortlessness of action.
You either like retro platformers and the style of game Derek Yu created or you don't! A lot of people don't. This is important because a lot of people seem to think getting people into a flow state will suddenly make them enjoy the game more. It's impossible to achieve flow without first enjoying the activity. Grinding in World of Warcraft may provide some players with an opportunity to enter a flow state, but designing the grind to be more conducive to such a state will never, ever make it more appealing to me. Flow is not a way to increase your audience!
9. People become absorbed in their activity, and focus of awareness is narrowed down to the activity itself, action awareness merging.
This seems to be the same as component 3. I'm not sure if I'm just misunderstanding one of them. The difference might be that this refers specifically to ignoring external stimulation, and is just a result of entering the flow state.