No syntax highlighting, no guidance on symbols to use, filters that are available - and no way to test validity without hopping elsewhere to mock up test data.
Even just having something that displays the current template value (like the template dev tool) would make a massive difference.
And a link to some succinct guidance on HA-relevant jinja syntax and avaliable data structures ( e.g. states.sensor…, trigger…, this(?) ) that’s accessible from a ? button next to the input box.
I’m so often trying to do quick automations on my phone and wrestling with flipping between qwerty and symbol input, automation editor, template dev tool, and multiple browser tabs covering jinja filters and syntax, HA functions and data model … it’s just so Exhausting.