This is an interesting recent discussion Number 1 mistake to not avoid during an interview Entire books have and will continue to be written on the subject. Personally I like the OA5 concept presented by Scott Adams.

Ultimately you need to promote an atmosphere that makes people feel valued and works to maintain employee 'happiness'. But you need to recognise that every single employees version of 'happiness' is going to be a little (or a lot) different. Google is one of the companies that seem to have found a good mix. If you want top notch programmers don't expect them to be happy doing code maintenence and UI tweaks. Most good programmers need continual challenges and will rapidly lose interest when not challenged. In fact as you observe they will actively go out and find challenges, even if they don't get paid to tackle them. Rewrite Perl in C++ by yourself, ask chip why not?

A mix of skill levels IS appropriate. Too many gurus are like too many chiefs and not enough indians. Not only that the more knowledge you have the easier it is to bog down in the minutiae. A balanced heirarchy potentially lets you take in lesser experienced individuals, train them up, let them see a progression.....

What you want to develop is a culture of enthusiasm. Enthusiasm for what it is that you are doing. What is hard is not so much finding it in the first place but actively working to maintain it. Enthusiasm combined with the skills to execute the task is a trully potent combination. It can also be infectious, just as one bad apple can quickly drain moral.