I just started playing Jarl, but have really enjoyed his style. I wanted to start posting how I would build a list around him one piece at a time. I have many pieces I like and will mention some things that stood out, but to start I will pick one unit and how it played out and what I think it will bring:
Support Light Warbeast
Impaler: A low cost light Warbeast that brings a no brainer Animus to the table = Farstrike (Snipe)
I have used this Warbeast in a lot of my lists with various Warlocks. With Jarl it adds even more as Jarl getting x2 16" Hand Cannon shots is huge! The 2nd best part is it can spread that love to so many other targets in a Jarl list. Add in Magic bullet and things get interesting - (Thumper Crew's Momentum shot + Farstrike + Magic Bullet + POW 14 = goodness)
In my last game the Impaler himself did not touch a single thing. In fact, he stood back behind Jarl and within 6" of Thumper Crew. It was pivotal in the game as both Jarl and the Thumper Crew would not have reached their targets.
** Edit: Just wanted to add, this is my own thoughts on building a list for Jarl and how I looked at it. No right or wrong, just one opinion to look at. Also, this is what I found my list needed under the constraints of being a 35pt build. At 50, things would change as you can spread out your beast options and troops etc.