Trigger expressions can be very long, because item keys can be very long. If you try to write them manually, odds are, you'll have some hard to spot typos. I find the expression wizard confusing to use and navigating to the necessary entities takes quite a while. Having a free-from trigger expression also suggests that you don't have to create the used items before, which is confusing for new users.

The {host:item} part could instead be replaced with placeholders, much like they are used for action conditions. That should make them a lot more readable too and increase the speed of writing trigger expressions.

{zabbix:system.cpu.load[all,avg1].min(300)}>2&
{zabbix:system.cpu.load[all,avg5].min(600)}>2&
{zabbix:system.cpu.load[all,avg1].time(0)}>000000&
{zabbix:system.cpu.load[all,avg1].time(0)}<060000&

could instead read

{A.min(300)}>2&
{B.min(600)}>2&
{A.time(0)}>000000&
{A.time(0)}<060000

Mind the "B", which is a lot harder to spot in the original form. This (modified) example from the documentation reminds me of ZBXNEXT-1719!

With some Javascript Fu this could be really fantastic. Maybe the view could be switched between the classic and the symbolic view.