Sometimes with stock photography it pays to 'play it safe' and add to your portfolio with similar images to what you have done in the past. Ultimately, though, I suspect that growing your portfolio in new ways helps a lot. Mostly though - it keeps you interested. While I don't have the luxury of spending hours on every shot I really enjoy it when I have an idea and take some time out to realize it. There is some risk involved as you could spend the time and then have it rejected!
For me it is very satisfying to get shots accepted when it takes some extra work to get the images completed. The recent assignment was an example of this. In addition to the assignment photo I had accepted and the one above spurred by the assignment, I also finished two other shots that were concept ideas I had written down and finally found some time to complete:
I always find that more often than not it is worth it to take chances - the sense of accomplishment seeing them online is all the more worthwhile!