The other answers to this are good, but miss some fundamentals. Here are a lot of reasons why almost any SaSS idea you can think of will take off if you can find a way to market it for less than you charge for it.
1. New business are opening all the time. Every. Single. Day.
2. Most businesses go looking for solutions only when something is very painful. Convincing them of the need is something a lot of existing SaSS businesses don't do as there's so much low hanging fruit around still. If you can send a salesperson in, you win by default. You'll be amazed at how much you can charge for something.
3. Lots of businesses (the majority) are 5-10 years behind in the computerization of their business compared to any business you find mentioned here or any business you'll ever speak to at networking events for startups. That's pre-SaSS.
4. There are a lot of businesses where the internal IT dept wrote all the 'apps', they usually suck and are broken. More and more businesses are giving up on this model and buying the app off-the-shelf. A little customisability (logo, maybe colours, even name) goes a long way.
5. A lot of businesses still rely on some crappy Sharepoint or even Lotus or [insert unwieldy enterprise system here]. The internal people hate dealing with them and will switch to your system if you find someone with enough clout to sidestep 'the rules'. A 'department trial' is also a handy way in, some companies have very autonomous departments.
6. There's no such thing as an established SaSS businesses outside of the echo chamber of HN. Many businesses will not have heard of Salesforce, let alone basecamp. There's no such thing as an established time tracking tool for example.
7. Some 'great' apps are far too generic. New to do apps still come out every month
8. Marketing and designing your app for a particular market sector can give you a huge advantage over others. Just knowing the right words, phrases and acronyms to use gives you huge credibility with that market. Compare 'Project Management' to 'Project Management for Architects with full support for Drawing Issues and Revision Tracking'.
Before you embark on doing anything though, you have to accept that any business app you write is going to be incredibly complicated and you're going to have to fight to keep it simple as everyone wants to use it oh so slightly differently.
Just to add to your excellent list, a lot of verticals have an established software company that's stuck in the desktop world (e.g VB6/Access/etc, CD-ROMs, licence keys, etc) and is often clunky to use or old-fashioned looking.
The difficulty with vertical markets is cutting through all the jargon/assumptions and figuring out exactly what the business logic should be. Everyone knows what they want, but very few people can actually explain it to the uninitiated.
Think Power Builder, etc. Our company has purchased software with a Power Builder front end (mid nineties interface) to an Oracle backend for roughly a million up front and $100,000/yr in licensing.
We've subsequently built a web front end to this app in house.
Sometimes there is no good substitute for clunky software.
You'd be surprised. I've had many coworkers who carried around dayplanners just to note the time and date they were asked to do something.
If you basically do the same job every day, showing up at 9 and leaving at 5, it wouldn't be for you. If you are working on multiple projects, it's a lot more useful. Especially if your employer has an SAP type time tracking system where the charge numbers are so ridiculously complicated. As a contractor for those kinds of companies, I've always struggled with keeping track of how much time I've spent on any given day among the projects I was responsible for.
Then there are other types of professionals, like lawyers, who have to keep track of multiple tasks on any given day for billing purposes.
Point being, there are still plenty of potential customers for time keeping apps. Maybe not enough to get rich, but enough to help make a living.
1. New business are opening all the time. Every. Single. Day.
2. Most businesses go looking for solutions only when something is very painful. Convincing them of the need is something a lot of existing SaSS businesses don't do as there's so much low hanging fruit around still. If you can send a salesperson in, you win by default. You'll be amazed at how much you can charge for something.
3. Lots of businesses (the majority) are 5-10 years behind in the computerization of their business compared to any business you find mentioned here or any business you'll ever speak to at networking events for startups. That's pre-SaSS.
4. There are a lot of businesses where the internal IT dept wrote all the 'apps', they usually suck and are broken. More and more businesses are giving up on this model and buying the app off-the-shelf. A little customisability (logo, maybe colours, even name) goes a long way.
5. A lot of businesses still rely on some crappy Sharepoint or even Lotus or [insert unwieldy enterprise system here]. The internal people hate dealing with them and will switch to your system if you find someone with enough clout to sidestep 'the rules'. A 'department trial' is also a handy way in, some companies have very autonomous departments.
6. There's no such thing as an established SaSS businesses outside of the echo chamber of HN. Many businesses will not have heard of Salesforce, let alone basecamp. There's no such thing as an established time tracking tool for example.
7. Some 'great' apps are far too generic. New to do apps still come out every month
8. Marketing and designing your app for a particular market sector can give you a huge advantage over others. Just knowing the right words, phrases and acronyms to use gives you huge credibility with that market. Compare 'Project Management' to 'Project Management for Architects with full support for Drawing Issues and Revision Tracking'.
Before you embark on doing anything though, you have to accept that any business app you write is going to be incredibly complicated and you're going to have to fight to keep it simple as everyone wants to use it oh so slightly differently.