Lighttpd

server-root is difficult to choose a default for. /srv, /var/www, you name it

/var/www is used for a single site, so using it for vhosts does not seem like a good idea.
I can't find /var/www in the FHS, but /srv is specifically mentioned:
/srv contains site-specific data which is served by this system.

Tip Rationale

This main purpose of specifying this is so that users may find the location of the data files for particular service, and so that services which require a single tree for readonly data, writable data and scripts (such as cgi scripts) can be reasonably placed. Data that is only of interest to a specific user should go in that users' home directory.

The methodology used to name subdirectories of /srv is unspecified as there is currently no consensus on how this should be done. One method for structuring data under /srv is by protocol, eg. ftp, rsync, www, and cvs. On large systems it can be useful to structure /srv by administrative context, such as /srv/physics/www, /srv/compsci/cvs, etc. This setup will differ from host to host. Therefore, no program should rely on a specific subdirectory structure of /srv existing or data necessarily being stored in /srv. However /srv should always exist on FHS compliant systems and should be used as the default location for such data.

Distributions must take care not to remove locally placed files in these directories without administrator permission. r20

There is no standard that is followed by most distros/software.
For such a important setting, it's not really a bad thing not to have a default for imho.
And what about other OSs?

There is no standard that is followed by most distros/software.

That's true, unfortunately. Why would that mean /srv would not be a good default?

For such a important setting, it's not really a bad thing not to have a default for imho.

What is the advantage of not having a default?

And what about other OSs?

The administrator would have to change the default.

A default that only works for some distros of one OS is not good.

What is the advantage of not having a default?

Making sure the deployer knows what he's doing :)

Why would it not work for all distros of an OS?

It's not about this being hard.
It's about improving consistency and simplifying the installation experience as much as possible. Getting the Install Lighttpd how-to as short as possible.

I think hiding configuration in default values is bad. If someone else reads the config he should be able to see these values.

Hiding these settings doesn't simplify anthing. If the user understands the options he will be glad to know them. If he doesn't, he will just copy+paste anyway, and for that some extra lines don't matter imho.

And there is no good reason to choose random values as default here. ("/srv"? "/srv/www" ? "/srv/http" ?...)

There are lots of default values that aren't in the conf. I don't see why these should be there.

/srv/<host>/htdocs has the advantage you could also have a /srv/<host>/ftp for example.

Nearly every distro/OS has a different place for web document roots. From /www over /srv/ to /var/www with different layouts in there aswell. No matter what you pick as default, it will be wrong.

Why would the module be enabled if default values are used?