This list seems very focused on that and seems to entirely miss asking questions like "is this cool or fun or entertaining"?
A common answer is that an existing product is great but it can be made even better or cheaper. It's weird to frame it as solving a "problem".
Lowering costs is solving a massive problem - for consumers and businesses alike. There are few greater problems to be solved for the majority of people, than lowering their cost of living - or cost of operations for a business - while providing the same or better quality of a thing.
That process has been a fundamental requirement for all of human civilization as we've known it the last ~5,000+ years. Without it, most of us are not here. It generates an incredible incremental gain spiral, a compounding of small surpluses that build and unleash bigger things over time.
For a business it frees up capital which can then be put into productivity, R&D, salaries and countless other things. For consumers it can boost savings or enable other purchases and investments that improve life. It enables new demand which will unleash new goods that were not feasible or viable previously.
It's all about generating a surplus that can then be directed toward some additional goal. Better and cheaper is one of the required tenets of mass standard of living progress.
A lot of ecommerce is basically this, your products are the same branded goods other retailers are selling, your website offers roughly similar experience and your pricing is roughly the same. Before the internet the differentiator was location, millions of store distributed across the world selling the same Nike shoes for example. Now a handful of ecommerce sites could serve the whole online market for Nike themselves.
As Rivierakid above says, it's kinda weird to frame it that way.
That's not right. People spend vast amounts of money and time on fun and entertainment hobbies education pleasure and leisure.
It's misguided to focus only on finding problems that need solving and pain that needs alleviation.
You'd think so but let me tell you - the hardest thing in the world is to convince someone to give you money for your product. You can easily get lots of compliments, lots of feedback, lots of positivity, but money? Oh boy. Just try and see how easy it is to find customers.
You are more like a musician.
One example is http://www.paulgraham.com/organic.html, but there are many others.
That things start out as toys can mean many things.
a) People create something for fun, it's fun and it turns out that after all, even if it wasn't intended, that thing actually satisfies a need better than current products on the market.
b) People purposely want to make something that's seen as mundane, boring, or annoying more fun. Lots of failed attempt at gamifying work here, but there must've been some successes.
c) People not so much create something that "starts out as a toy", but they create a toy, plain and simple. It's an entertainment product that is particularly well made and people like it because it's inherently fun.
The underlying idea is simple: if we take an optimistic view of things, things that people do not inherently enjoy will either be entirely automated (and "disappear") or be turned into things they inherently enjoy ("toys").
So, if you want endless sources of ideas, think about those:
a) How can you automate things people don't inherently enjoy (probably 90%+ of existing jobs)?
b) How can you make things people don't inherently enjoy enjoyable?
c) How can you make things people already want and enjoy (entertainment mostly), but better?