Spam protection. Paul Graham's article recently spurred discussions on best ways to eliminate spam. Donald Knuth has provably optimal solution: get rid of your e-mail address. Downside: it only works for people whose role in life is to be on the bottom of things.     

Aggregation is not enough.The way you define problem determines how you'll approach solving it. Sometimes that makes a big difference. Let me give you an example: writing RSS aggregators is now en vogue (I see a new one every day). But those programs never go out beyond solving "I want easily aggregate RSS feeds". But is getting RSS feeds really a core problem? No. The problem is bigger (and less well defined): we want to get new information on topics of interest to us. RSS feeds are a partial solution, but it has weaknesses. You have to actively look for feeds that match your interests. RSS feeds usually cover more topics than you're really interested in but the burden of filtering uninteresting news is on you. What would happen if people tried to write "news gathering/filtering software" instead of "RSS feeds readers"? In my opinion we would get better software.

Practical tip: before solving a problem think a bit to find out if there isn't a larger problem wanting to be solved.

And here's a list of RSS aggregators:

• Headline Viewer
• AmphetaDesk
• Radio UserLand
• PluckyX Headline Grabber

(and more; I'm not creating exhaustive list, just making a point).