Working on something every single day is hard. There are only so many hours in a day, and the commitment to do one thing at the exclusion of something else is a big one. This gives system designers an extra challenge - they need not only to engage you while you're working on the system, but also to give you enough fond memories of the system while you're away from it that you'll voluntarily come back to it day after day.
Part of the challenge is to minimize the friction involved in getting back to something day after day. If you have to go through an elaborate login sequence, it will be all that much harder to want to get involved. Any time wasted in the process is not just a few extra seconds one time but a daily commitment to spend that time, over and over again. Good daily systems have a minimum of rough edges and very little to get in the way of that habit.
There's an element of gaming in 750 words, the process of accumulating a score and badges that reflect progress that can only be achieved over a long period of daily commitment. Until you get three days in a row in, your system icon is an unformed egg, and the number of people who stay in egg state and never graduate to the next level (a turkey) is substantial. As you grow through the effort of daily activity there are opportunities to decorate your account with more and more bling, status markers that give everyone else a chance to recognize you and distinguish your account from all of the others.
Naturally, one good way to get added status is to pay for it. Gaming a system quickly leads to monetizing it, since you have a social feedback mechanism that allows people to recognize not just diligence as a sign of their specialness but also the willingness to shake loose some coin in appreciation. So the pattern is simple: build a game that you play for a few minutes each day, reward frequent players with visible signs of their progress, and encourage the devoted ones to add an additional reward that's easy to purchase and distinctively visible.
I've been motivated by online games before, but then got to the point where I had to give up on them because the time commitment wasn't worth the effort spent getting bits of online flair. I appreciated the feedback and liked leveling up but just couldn't commit beyond a certain level to care more about the whole thing. DuoLingo, for instance, was great for a while (and I got to level 11 in German, woo) but at some point it got to be too much.
There's a lot of dropout in massively online open courses (MOOCs, as they are called). Hundreds of thousands of people will start something, but only a few thousand will finish. It's very easy to start a multi-day project, but the commitment to come back day after day to finish it is an order of magnitude more exceptional.
The alternative to games within systems as a method for reinforcement is to use external rewards from other people. Linda Diane Feldt has talked about how wonderful it is to have used the Android app CardioTrainer to log her workouts. It simply keeps a record of what you have done and records it to Facebook, and doesn't appear to have an internally exceptional set of elaborate games to prompt you along. What the Facebook connection offers is an opportunity for other people to provide intermittent reinforcement, the random and unexpected "yay" for a good workout that isn't pre-programmed into the system but comes heartfelt from other people.
This is only my second day into 750 words, and I'm not yet at a point to judge it. I like accumulating points as much as anyone, and I like that I can spend a small chunk of time writing instead of playing some other online game and that the result of the effort will be tracked and calculated and commented on favorably, at least by a machine. From time to time I expect to cut and paste these lovely long essays into my weblog after they get a round of edits, and it's from that effort that I look for the intermittent reinforcement that will most likely keep me going - not that the "turkey" badge isn't motivating to get through the first week, just that in the long haul the real rewards come from other people.