I, too, am happy about Mundo buffs. As far as Vladimir goes, Riot is trying to find a way to make his early game less difficult. With a short early CD on Transfusion, the lower cumulative cost on his Tides, and his seemingly costless Hemoplague (the most WTF change of all), he definitely has more of that. As far as Sivir goes, yes, she is recognized as one of the stronger AD carries, if not the strongest. And with the nerf to Graves and Vayne, the other top carries, Sivir being the strongest isn't an unreasonable claim.
INCOMING SEJUANI RANT:
You know, Sejuani didn't seem all that appealing to me until I saw her spotlight earlier today. I think the things that sold her for me is her Q going through walls and her ult's quick projectile speed (a blessing if you've used champs like Morgana, Olaf, or Galio). She's essentially a slow bot, which is actually not too shabby. If the stats I read somewhere are the actual stats of her kit, then she should be able to apply an initial 35% slow for 3 seconds with a level 3 gank (Permafrost + red buff). While hard CC is preferred, I can see people being wary of extending even a little bit if she's on the enemy team unless you have your own gap closer/Flash off CD. I'm thinking along the lines of Lee Sin, but without the sheer damage.
Her W scaling with her max health, as well as having increased damage if the damage is being dealt to those afflicted by her passive, means that she'll still do significant damage while being a pure tank. Her shortest CD seems to be 10 seconds, so I can see how Frozen Heart would be great on her. Frozen Mallet will be core, most likely. She just seems like she'll be an extremely effective tank, so yay. Would she build Wriggles like other champs though? Her abilities are pretty strong, but then again, lowng CDs, so probably. I just wonder if she should worry more about items like Wriggle's and Wit's End, or not bother with the on-hit jungle items at all.
In terms of utility tanks, I guess she's most comparable to Leona? Leona has the hard CC Sejuani lacks, though depending on the AoE radius of Sejuani's ult, she may win there. In addition, Leona is tankier thanks to Eclipse, and can keep a single champ locked down with her stuns. Sejuani, on the other hand, has superior damage output thanks to the health scaling of her W, and can apply significant soft CC to multiple champions. To be honest, I would hate to go up againt a team with both of them; you'd be hard pressed to reach their squishies without some insane mobility of your own. The first champ that comes to mind is Fizz, who can E over her charge and Q to his target.
Now I just wish I had the RP to pick her up...perhaps I'll invest in some, since I never have any anyway.