Sure, I suppose it matters a lot on the type of Web application you're building. If you have a relatively simple domain model with very few relationships, I'm sure a KVS works pretty well. Still, for anything other than applications with simple domain models, or ones that need tremendous performance, a KVS is inappropriate outside of isolated optimization -- in practice, one runs into headaches with non-normalized data very quickly (think MBAs and their giant, messy Excel spreadsheet).

I disagree.

In most web apps data in per-user (or per-object, whatever that object is), and it's not a major headache to denormalize it to be per-user. One notable is exception is a social network, but that can be handled with some care.

We're talking about web apps written by programmers, not MBAs. Sure, if your web apps are written by MBAs, then they should use an RDBMS, because they can do less damage (?).

I'm not arguing against RDBMS/SQL here, all I'm saying is that a KV store is actually a good choice for the domain of web applications. You can also use an RDBMS of course, as we have for the last 10 years with the LAMP stack, as long as you're handling a managable amount of traffic. Even with a lot of traffic you can handle it with an RDBMS, it just stops making sense at some point.