I wish I would have focused on more profitable ventures. For example, I've spent a ton of time on my blog, outside of a handful of the blogging book sales it really hasn't made much money.
But.. these low revenue projects have given me really good insight, knowledge, and contacts. Even in retrospect, I don't think there is much I would have done differently.
Another little thing I've noticed is that the high profit stuff tends to only work for so long, then other people figure it out and do it. The lower profit stuff tends to result in more stable & long term revenue. For example, last year I dropped $3000 on a premium domain and started a forum. The revenue from it so far has been pretty mediocre for the amount of time and effort I've put into it. But, I've built a base of very loyal users around it and its been growing by word of mouth. 5 years from now, I know the site will still be around and making money (assuming I haven't sold it.)