On the PBE forums Riot has stated that Sejauni isn't a 'true tank'. Nevermind that her defining feature is building HP. Nevermind that she was the Poster Child of the "Tank Meta" when Cinderhulk was overturned. They state that because her getting ahead doesn't ' help set up your team', she isn't a tank.
Except in my opinion she DOES set up the team, or get set up by them. Her Damage is rarely enough to outright 1v1, her damage is meant to soften so that your assassin/burst can 70%-0% them, or capitalize on a low hp threat by finishing them off. Her set up 'scaling' is that she is beefy enough to survive, and that she locks down and deals 'enough' damage to make finishing a target good, or follows up. I don't want Riot to say "let's give her a Current HP or Missing HP nuke", as I like the fact that she has the option to either initiate or follow up,
I personally love Sejauni the way that she is. I love the fact that she can be viable at all stages of the game, early, mid, and late. I love that I can both be an effective team fight champion while at least standing on my own in 1v1. I love that I can "Carry With Tank", and that it isn't inherently broken now that the numbers are tuned correctly. Her major weakness is that she can't actually deal with consistent DPS if she's peeled off/kited, or the enemy actually has enough beefcake to get past her cc rotation, then utilize their own dps (Devourers anyone?).
To me, Sejauni is the lockdown initiation tank that is moderate in early game, and snowballs into late game or if she gets ahead. She builds a boatload of HP, Counters burst and ultra squish champions, and gets countered by enemy dps that doesn't die immediately.
So, "What Is Sejauni" to you all, personally?