1. Startups have happiness potential for developer not only because of the monetary wealth but also because of the wealth of the challenges faced. Developers live for tricky puzzles to solve and startups are one place that you can get them. Startups aren't the only place where you can find challenges (as you've noted). The monetary wealth typically comes from stock options. Stock options are a promise that you can bu…
Ehhh. I jumped from research to the startup world, so maybe I have a high bar, but most startups are doing pretty boring, predictable variations on bog-standard systems work. You see this in the way that these "exciting" companies keep re-inventing the same wheels in different languages. How many javascript frameworks are there, again? How many different ways have people found to "replace" relational databases?
If you're right out of school, everything is challenging. Give it a few years. Eventually, it becomes downright boring to watch people needlessly re-solve problems that were solved in the late 1960s because they don't know history, and/or don't want to learn some boring old technology that their older brother used last week.
If you want interesting problems, go into research. If you want to do interesting problems and get paid, go to companies that are large enough to have the resources to fund things that aren't directly on the critical path to profit. Startups are a really bad place to go if you want tricky puzzles.