I admire Ralph Nader. A little bit. I've never voted for him, and I would not. What I see as his biggest weakness is an inability to effectively forge coalitions. Why is he a perennial Presidential candidate? Why not seek to join others, and upgrade a local, state, or national platform? If he can't work well with others, he shouldn't be trying to run the country, because he can't do it alone. It is sometimes useful to snipe from the sidelines, but running for President is perhaps a poor vehicle for such a person's energies.
I'm also disappointed with both Nader and McCain for not understanding / recognizing the benefits of computers and the internet. I'm OK with slow food and walking to work, or people limiting their mobile phone usage. But access to the World Wide Web is great - those who don't understand it are missing a lot.
Nader's weaknesses
McCain's weaknesses and more here on McCain handicaps