I was messing around with Alexa yesterday (after finding Movers) and decided to compare del.icio.us, BlueDot and Ma.gnolia. The first thing that struck me was the magnitude of difference : del.icio.us is way ahead of the others for traffic. This seemed particularly interesting to me because I find BlueDot much more useful than del.icio.us. To quantify that a little I can find more useful links in ten minutes with BlueDot than I could in the same time in del.icio.us. Part of the reason for this is that BlueDot offers an easier to use system with more useful functions. BlueDot also seems to have a more targeted selection of users. Hence I am finding other users dots more relevant for me.
Ma.gnolia is another great looking system. But one reason I've never gotten really interested in it is I haven't found any useful links in Ma.gnolia. It is well designed and easy to use. But the target audience is less interested in web development issues. Hence Ma.gnolia is less useful to me. But I imagine if more people started using Ma.gnolia then it may become more useful than BlueDot. Of course, having more users can quickly point to the limitations of a system. The biggest problem for del.icio.us is success.
For del.icio.us to become more useful it would need to find ways to simplify link finding within it's system. Because the two real factors that will determine the most successful system is the number of useful links for a specific target audience and how efficiently those links can be found.