Base health increase screwed botlane balance
Last season strong lategame carries like Vayne were kept on bay early on due to kill pressure from other carries like Cait, or even Sivir when combined with a champion like Leona.
With the base health increase the kill pressure got significantly reduced.
On the bright side this balances out champions like Tristana who just had a little bit too much kill pressure at level 6, on the other hand it's significantly more difficult to keep champions like Vayne from becomming true monsters.
Please consider improving Sivir and Varus and give them back some of their early lane presence, or make up for loosing this advantage.