As we thinking about migration from nagios - the one thing we really miss - is the parent-host relationship. Not to say it's not perfect in nagios, but in most cases it's more than enough.
As a network administrator in ISP when installing new hardware i know from which node/switch it will be connected - so i just write in one time, and then forget. If i want to know some time later how things are connected - i just open nagios, not the paper documentation.
In zabbix there is IT services, yes. But it's more work to do. First i create host. Then i create trigger (ok, trigger is templated). Then i go create service and assign it. And there is no clear way to see realationship of the hosts.

So what about adding parent-child specification as extra field in host configuration (optional field, of course). Or provide way to template somehow it services show in host overview parent service (host)