If your goals are to gain experience and make money going the startup route, you don't HAVE to be the one with the idea if you're capable of implementing the ideas of others. For best results, you just need to be able to RECOGNIZE a good idea that is presented to you and generate enough passion about said idea that you can make it the focus of your life for awhile.
There are people that have the opposite problem that you have...they have tons of good ideas (and maybe contacts and money), but lack the skills to implement them. Find those people. Start a company with them. Rinse. Repeat. Eventually, you'll tend to rub off on each other and you will gain skills in generating good business ideas on your own...
If you copy something that works and then add features or put a spin on it you can end up with something new and cool. Its a good way to start thinking about ideas imo.
Isn't Digg just google news or nytimes with new innovative features?
The biggest mistakes you can make, IMO, are (a.) thinking your idea is too stupid to start with, and (b.) keeping it a secret. The idea you start with almost certainly won't be what you finish with; the important thing is to get started. And if you don't talk about it, you'll never really explore, refine or (as you'll almost certainly need to) abandon it for something better.
Most people have something interesting to say. Find out what it is.
Good Luck
Write the ideas down. Look over your list. Pick the one that seems the most interesting AND has the best (clearest/easiest) method of distribution.
For instance, Gmail is a MUA with good spam filter, keeping e-mails in threads. So, why not create a blog reader that keeps discussions between bloggers in threads, making it easy to follow?