I mean, I'm slightly biased because I'm the one who suggested it and got the ball rolling on implementing it, but I think the new complex operators are very cool and incredibly powerful! (PRs here and here for the coder types)
Essentially they massively overhaul how expressive you can make your templates, rather than the usual "replace this bit with a value I give you" functionality they already have, the templates become a mini formatting language.
For example, if you had a template for taking meeting notes and have a variable number of headers, you would have to use this previously:
---
topic_1:
label: Header 1
type: boolean
topic_2:
label: Header 2
type: boolean
topic_3:
label: Header 3
type: boolean
---
{{#if topic_1}}
# First Header
{{/if}}
{{#if topic_2}}
# Second Header
{{/if}}
{{#if topic_3}}
# Third Header
{{/if}}
The thing I found annoying about this, and what prompted me into looking into improving it, is that when you create a note from this template, you have a yes/no box for each header you want to include, as opposed to what would be the (imo) sane approach of having one dropdown to pick a number of headers, then the template would figure it out from there. After the feature was added you can do that, with the below:
---
topic_num:
label: Header number?
type: number
---
{{#if (compare topic_num ">=" 1) }}
# Header 1
{{#if (compare topic_num ">=" 2) }}
# Header 2
{{#if (compare topic_num ">=" 3) }}
# Header 3
{{/if}}
{{/if}}
{{/if}}
Initially this wasn't planned to go any further than basic numerical comparisons, but @nishantwrp added a bunch more little helper operations so you can now do stuff like this:
---
topic_num:
label: Header number?
type: number
---
{{#repeat topic_num}}
# Header {{math repeat_index "+" 1}}
{{/repeat}}
(Every thing below here is just to show off the features)
{{#repeat topic_num}}
The square of {{math repeat_index "+" 1}} is {{math (math repeat_index "+" 1) "**" 2}}
{{ repeat_index }} is {{#if (compare repeat_index "!=" 2)}} not {{/if}} equal to 2
{{/repeat}}
Today is {{ date }}, tomorrow is {{ datetime set_date=date delta_days="1"}}
So now you can do all kinds of things, like maths operations, logical comparisons, string case changing, powerful datetime manipulation, etc
(this is also why writing the docs is a big and legitimate hurdle to the next release (though since I'm fairly familiar with this stuff I might do it myself and make a PR if he hasn't appeared in a month or so))