It takes longer for her to burst someone down now because R is more auto-attack based and the burst has been lowered. It's now HARDER (a lot harder) for her to Flash Tibber Stun a whole team or half the team because of the reduced radius.
As someone who's played lots of Annie in Ranked before, this is very noticeable and so I would consider it overall a Nerf (since 6.9).
Her 1 vs 1 in the lane is better for some match-ups (since Tibbers is likely to be able to stay on the target and get out more damage over the engagement) but for many match-ups she's now weaker because of the new mechanics (ex Akali now has time to outplay Annie by turning invis against Tibbers whereas before she would be bursted dead immediately).
Yes she has lower cool downs now (largely due to 6.9 item changes) but damage goes down dramatically if she goes for CDR route (really, there is no choice but to go Rabadon either first or second if you want to have relevant damage). Yes she has Hextech Belt (30 second dash) but it's not as powerful as it sounds and if you build it first or second, you lose quite a lot of pure AP so its questionable.
Overall... she's harder to play now. Her multi-target AOE potential has gone down and she's now more of a 1 vs 1 champ. You have to think about when to go in and its more about SEQUENTIALLY being able to take down targets then ''how can I burst down their entire team or half of their entire team.''
You are rarely able to take advantage of E's new mechanics because its still your bread and butter ability for charging your Stuns. You rarely have it available when you need it since its so often used prior to a major engage to charge a Stun.