Startup work is mundane but still really rewarding because you live and die by your own abilities. Besides, if you make the startup work you can go work on any hard problem you want because you've removed the 'I have to make enough money to pay the bills next month problem. PG did it, they build Viaweb and once they were done he started working on Arc. Nobody pays you to write new programming languages, and I'd bet money the person to make AI real won't be working for Google. The person who solves that problem will do it out of love and determination, not because they wanted to work at the Googleplex.