I'm currently using Mandril, the things I miss the most are template versioning, some logic into them and a WYSIWYG editor (so people stop asking me to change the emails every 2 weeks).
We change the templates a lot and I need to create another version to test and keep the old one for the production, it kinda sucks;
Conditions and loops inside the templates would be awesome, Mandril actually has it using Handlebars but it's still buggy (at least it was the last time I tried);
WYSIWYG editor: the marketing staff sadly don't know HTML...
That sounds like a nice use-case, being able to version templates, I'll add that. Conditions and loops is something us Java devs are spoilt with in templating engines, I've even built my own templating engine that allows conditionals and loops.