currently it is only possible to add comments to triggers.
it would be also useful to add comments to other entities :
items (for example, cautioning against too short intervals, explaining needed permissions on agent side etc);
templates (explaining it's usage and impact);
hostgroups (adding general group contact info etc).
ideally, this would be coupled with host inventory, so that these entities might gain multiple fields instead of a single 'comment' field.
in that case, ability to customise profile fields would be mandatory - ZBXNEXT-336