Would be nice to add additional internal check like "percentage of pollers used".

Currently it's quite hard to detect that all pollers are blocked (waiting) due to time-outs (which may happen if you have a major problem in your network).

In this case Zabbix gets quite "unreliable" as you get no alarm or indication what is going on and why items are not monitored properly any more.

A solution could be an additional item for the zabbix server like "percentage of pollers used".