Let me be the first to say this but I hope that there is a generous grace time before that sun ray falls.
Positioning has always been a strong point of LoL where as in Heroes of Newerth, abilities and items allow heroes to disengage or counter in an instant. This is why characters such as Amumu have always been high ranking picks. Engaging in League of Legends normally consist of vying for a vantage point laying ambushes or blind firing an enchanted crystal arrow. Bubbles is not in this game and cannot simply roll across terrain and initiate a fight. Flash is too valuable a skill to waste and the CC is not enough to hold them down and quash focus fire. Leona is a tank, femininity aside, whose ultimate can slow as well as stun in a global area-of-effect nuke.
While she is still in development and the only information source I have would be a short E3 spotlight, this sounds incredibly dangerous. We’ve seen the power of globals, Twisted Fate has always been a strong pick. Shen, Karthus, and Ashe are all solid champions to play. I have no idea what her other skills are to be but this ultimate seems to be too sweet in my mind. It widens the number of tactics available from baiting Baron Nashor or slow pushing mid. Split pushing and ward control have become increasingly popular as stalemate fights are never any fun, it slows down the pace of the game. A giant ray of sunlight crashing down upon you from any point on the map will be a strong threat if used correctly and can break up the monotony of waiting in a bush for minutes at a time.
The current meta-game is to run two ability power casters, one squishy and one tanky, have a support plus attack damage carry bottom, and a beefy jungler capable of some form of CC. Previous to this was the two tank bottom lane, tanky jungler, tanky top lane, and AD carry middle. The balance is all lop sided and I believe that Leona can help to address this issue. Vying for position hoping to pick someone off is nice as it requires a decent amount of skill to plan and execute an assault but having a character capable of globally stunning changes the entire meta. A balance of counter-initiators and walling tanks will be needed to stop how powerful this ability may turn out to be.
Rarely played characters such as Maokai, Dr. Mundo, or even Kayle could be utilized to great effect as disengages. A Leona ultimate perfectly targeted may not be used to stun but to disperse the enemy forcing them to regroup. Fat characters or characters with damage mitigation abilities can be used to stop the inevitable charge.
Leona can perhaps change the entire meta. League of Legends has such a wide array of playable champions and yet some are obviously better picks than others. Perhaps her presence will allow for even greater diversity in team compositions. The fact that she’s a female tank is just icing on the cake.
tl;dr – Leona has a hard initiate which will break up the monotony of waiting for a pick or ambush, specialized characters will be needed to counter engage providing a more diverse initiation strategy. She’s also a female tank. ;D