Poppy on Live is known for two things: her passive that prevents her from getting destroyed within seconds and her ultimate that lets her go after important targets on the enemy team. On live she is an honorable and tough sounding warrior that isn't usually befitting of a Yordle; now she sounds like an extremely modest young adventurer looking for a hero of some kind.
She is less tanky, depending on her passive that is almost useless in an actual fight because it flings so far away. Her old passive made her amazing against heavy hitter and assassin comps that didn't have a lot of DoT damage. Her ultimate also gave her complete immunity to everyone but her target.
She has less damage. Before she would be able to build Trinity Force and E-Q someone for that massive on-hit damage boost. It also reset her autoattack timer letting her AA-Q-AA for really good damage. Now her Q needs to hit twice to deal any good damage which almost always requires you to stun the enemy into the wall with her Q. Not to mention the scaling isn't all that good, seemingly around 10%-20% on each hit. Her current Q has 100% AD scaling and 60% AP. It was a nuke and what made Poppy fierce; something she has apparently lost in this rework.
Riot literally kept her charge practically the same because it is apparently what makes Poppy, well, Poppy. That isn't true at all. It was a part of her wombo-combo that can lead to the deaths of enemies, or you living through kiting, but her Passive and Ultimate is what she was known for, not a charge that stuns. If you want to say a champion's identity is based off of something like that, say it is Vaynes, but not Poppy's.
Her ultimate. Oh god I wanted to love it, but god I hate it. It is an amazing looking skill that has a lot of good uses, but those uses can very easily turn into an advantage to the enemy team similar to a Bard ult. The damage is EXTREMELY low for any type of finisher forcing you to get the enemy below 200-300 HP to kill them with it. Max damage is (if I remember correctly) about 400 with tiny scaling. I couldn't even kill a Brand Bot (not going to build Armor) with 300 HP with the skill with well over 200 AD. It also has a ridiculous CD at 140/120/100 and if you get disrupted or not cast it at all (which you must hold down the key for, if you release it even without a target it goes on CD) you will have it go on a 30 second CD. Screw up once, or get stunned/knockedback (Janna is one of her counters according to riot) you've lost a large potential tool for a long time.
Overall I do enjoy her, but I would enjoy her more as a NEW Champion, but not as a rework over a previous one. Sion was known as a pure Juggernaut. A massive warrior that drenched himself in his enemies blood and a vicious warrior. His rework complemented that and made him absolutely amazing (Riot screwing up nerfs kinda soured the rework, but that is another topic). But Poppy went to a fierce, battle hardened warrior to a small, giddy girl looking for adventure that throws credit to her hammer. It is all "hammer this hammer that" with her. I just don't feel like a badass and a giant middle finger to the enemy team, I just feel like an Inconsequential part of my team and something the enemy team can easily target because my ultimate will just save them if I use it and I'm out of a passive during a fight.