The key to happiness when working for someone else is the happiness of your co-workers. Specific technologies and projects are just a minor consideration. Happy people can be a sign of decent management, decent hours, decent pay etc. Not a guarantee but a good litmus test.

Unhappy co-workers can ruin anything. Doesn't matter if your job is printing money on a yacht in the Caribbean. No amount of sunshine, drinks or swimming can chase away the awful feeling of seeing the same unhappy people day after day.