What makes a web site (or a web app) a success or a failure? Having seen a lot of successes and failures I find there are a few common types of business website that have different risks and determinants of success:
A business can be destroyed by losing its data. I have known even losing the data on a single person's hard drive to be very expensive, and losing your servers can be much worse. You need to think about backups, continuity, hardware. I have taken a server out of a bomb blasted building and it worked - but it might not have.
Technical debt tends to be talked about, and worried about, more by developers than by those running a business or looking after its finance. The problem is that not taking it into account will lead to bad decisions.
Most people (out side the industry anyway) assume that web sites should look good, and an aesthetically good website will produce better results than an ugly one. This is not true, and many of the things that make a website look good will tend to make it worse for users.
SaaS (software as a service) is extremely popular right now, and it can solve a lot of problems. It means someone else takes responsibility for maintenance and security. Of course nothing is ever that easy a win.
So, what are the problems? Our experience is that there are three, two of which are closely linked: