This doesn't mean you don't iterate. A lot of the lean startup stuff is still useful to us (a/b testing for more focused UI etc.). But generally speaking, we know where the product is going, and our customers' needs, often better than them.
You want to validate your business assumptions sooner rather than later and there's nothing wrong with that. You should also tweak your business based on the real world. Nothing wrong with that. Nothing really that new about that either. Other than that maybe Pivot is the new Agile?
What SV does have is a very large percentage of the successful internet start-ups. But there are so many more lines of business where untold billions are being made by companies that go for non-sexy non b2c approaches.
What works for YC and what works on the web is a very visible very thin slice of the software world as a whole. In a way you're making your life harder if you pursue that particular niche because even though that's where all the easy PR and the most visible success stories are it is also where the competition is fierce.
B2B is a lot less likely to make you billions. But it is more likely to make you tens of millions. So if you want to reduce the luck factor and increase your chances of success then b2b would seem to make good sense. It also works better if you will have to work without access to capital or a presence in a place like SV.
Way 2. Mix with people in the real world that work in the companies above. Listen to their problems, offer solutions, offer to pop in after work one day or on a weekend and look at their problems.
There are a ridiculous number of them out there.
The only problem is it's a lot easier to post, comment, kibitz, criticize than to actually do. It's nice to read the good advice here, but at the end of the day it's still necessary to do something useful in a startup. This isn't a criticism of your post, it's a general observation.
In my country, faxes are legally recognized as evidence, but email isn't, so there's a strong incentive to fax documents.
A law recognizing electronic signatures might help things, but it's still pretty cumbersome.
I tried to build a solution for "escribanos" - the U.S. approximate equivalent is notaries, but they're specialized lawyers actually - and the legal hurdles proved impossible to overcome.
Stuff that is being done with fax machines out of ignorance would be a better place to start, but there you have to face conservatism (an admittedly easier barrier than legal obstacles).
The best entrepreneur IMO brings a combination of the technical expertise with understanding of the problem, preferably a personal/deep understanding. If you want to understand specific business problems for some specific businesses/industries you need to read about them, talk to people from the industry and/or work in the industry. Some problems are generic and cut across industries. Some b2b problems are the same problems individuals have in performing their work.
Some problems are easy, e.g. have easy solutions, everyone knows about them. Some problems are hard, a lot of work to solve. Some problems are very niche/specific, no one knows about them...
In a month you will know which problems stand out
Start marketing around that problem, build an audience, evangelise something valuable.
Take your own good advice :-)