Sthenno told me awhile ago about an interesting challenge he went after in Starcraft 2. The idea was to play through the campaign building *only* the special units that appear only in the campaign on Hard difficulty. This is really tricky because the units that only appear in the campaign are pretty much all completely trash. It isn't winnable on Brutal I think but Hard is doable. The roughest part so far has been getting started since I am restricted to using only Firebats. Firebats have this problem that they do very little damage, have low range, can't hit flyers and are big so they have tremendous trouble concentrating fire. Most missions have the problem that although Firebats can kill off much of the enemy force eventually a flyer shows up and then I have to beat them with the two Marines I start with. It is possible to aggro flyers with my Firebats and drag them across the map to a well placed Missile Turret but it isn't feasible when the mission is actually challenging otherwise.
I ended up doing a really complicated dance trying to figure out which missions I could actually beat by spamming Firebats, the most useless unit known to Terrankind. Though I have already beaten all the missions several times trying this bizarre little twist really got me back into playing the game and made it fresh and new again. I had a blast trying to figure out how I was going to beat Xil in particular because normally you beat this mission by stacking up a billion Siege Tanks and ruining the enemies. Not having access to Siege Tanks makes it really quite tricky though, especially since the only units I was allowed to build at this point were Firebats and the execrable Diamondback. I ended up building a full 10 Planetary Fortresses and plastering the world with my base from wall to wall. This is exactly the sort of gaming experience that I love, experiencing that desperation, that rush of hunting for solutions and stretching to find a new, creative solution to a problem. Most of these missions have really obvious solutions that either include Medic/Marine or 'Spam the new unit for the mission' but when you aren't allowed to build the new unit you have to search for some other way.
I think this illustrates very well how important it is that bad decisions exist within a game. I have said it before and I will say it again - it is critical for a game to have many ways to play well and even more ways to play badly. It is of course not true that randomly penalizing yourself makes a game good but sometimes one can find a strange way to play badly that otherwise makes the game really tough and interesting. It could have been that the arbitrary restriction I chose ended up being either impossible or utterly trivial and neither would have been much fun but I got lucky and there ended up being a tremendous number of challenging maps and difficult decisions. Somehow I got in the sweet spot where there were both a bunch of tricky strategic decisions and a bunch of great moment to moment gameplay.
Much of the replayability of single player games comes from this source. People seek out strange ways to play their games and go through them multiple times under their bizarre restrictions. Some of them are good, like this one, and some are bloody brutal, like when Ziggyny decided to beat FF1 with just one Thief. Not that the Thief challenge isn't challenging, it was, but 20 hours of mindless grinding in the middle really kicks the fun factor in the junk...