Anyway, as a proud Diamond Darkrider Sejuani player, here's my opinions on what Riot currently seems to have in store for my precious:
Slightly less AD and armor, but a bit more HP. Nothing drastic here.
Passive: A lot better now. All that changes here is that you should get Permafrost on level 3 instead 4 all the time rather than most of the time. +10 armor and slow resistance, old passive moved to E. Straight buff here.
Arctic Assault: It deals extra damage based on T_MAXHP. In an area if you land it right. This is a massive buff and turns your 1-point utility wonder into a beastmode damage ability. You can still leave it on level 1 early-mid because the damage increase is only really noticeable later on when people have more MAXHP. Mana cost and CD favor leaving the ability on level 1. Will have to check if the slight range decrease stops you from jumping the thick Wraith wall.
Northern Winds: This is a QOL nerf - you now have to be in melee range to damage things and can't just walk by them to kill them. As for the numbers...
16% of bonus HP gives you +160 damage per 1000 bonus HP. Before, you had 3% of C_MAXHP bonus damage on Frosted targets per second - that's potentially 18% of C_MAXHP total damage, obviously quite a bit more than the new version to someone getting hit by all 6 ticks. Even the AP ratio was better before, by 0.3 to Frosted targets.
However, taking the first hit into account, the AP ratio evens out and the damage is higher over 4 seconds but falls off against a target that would have taken the full 6 seconds of old W.
It's too early to call it, but by numbers alone - the ability is slightly better over a 4-second period and weaker over 6. Also, you need to be in melee range as opposed to slightly out of melee range, at least for the initial hit.
This also helps you jungle, since you do more focused damage to the big target and don't have to waste time applying frost to weaker targets (not that you really had to anyway).
I'm gonna take a leap of faith and call this an overall buff.
Permafrost: Duraction scales per level, but the ability is a lot stronger at early levels with the stronger slow, which means it evens out on the distance your target can travel while affected. Also means you can now choose between leveling Q and E...though E is still a far better choice IMO. More slow > slightly more damage.
Glacial Prison: Nerf early game, huge buff late game. Thing here is, you actually have to hit the ability now, rather than "almost" hit, which I did pretty often to increase its effective range and catch people off guard. But 2 seconds on everyone is a big deal and a lot better than 2 for 1 and 1 for the rest - just means it actually matters how well you aim it now.
~Victory needs no explanation; defeat allows none.~