Tiger's accomplishment represents a week of very good putting but it's not so unbelievable. I once played a 54 hole tournament missing only one putt inside of 10 feet, and it was a 9-footer. Obviously I putter great that week, but if I can do it, I can only imagine that Tiger can as well! Actually, Doral's greens were similar to the greens I did it on in Las Vegas - bermuda, smooth, and fast without being scary.
Making the greens a lot faster would make them harder, but then they'd just have to be built flatter. Increasing the slope would make them harder but if they're smooth, the pros will still make putts, especially because the speed would have to be only moderate.
I think that, from time to time, people just simply putt that well.
The one type of course I couldn't imagine accomplishing that kind of feat on is one where the greens are all domed/crowned/convex. Those are by far the hardest greens to putt, regardless of speed.