So I've had enough time with the Update to be able to voice my opinion on it. For the most part I'm thoroughly pleased with it so far.
Action Center:
I have no real problems with the Action center aside from not having Battery Saver pinable to it. Aside from that I just need to get used to using it. Whatsapp shows notification from the last one sent forward, which is odd cause you have to read the messages backwards sometimes to understand. Its not a big deal but if I'm just reading it through the Center with no intention to respond it gets a little confusing.
Cortana:
Again, I have no major issues with it. I'd like the ability to edit the reminder sounds per Reminder, but that's not a big deal. I'd also like to see more from the entire thing(being able to do more than one task at a time, using the voice comms without touching the phone, etc.), which will come in time. The only real issues I have is just bugs. She'll sometimes not hear me if I don't speak loud enough, and the system itself is a tad slow for doing just normal searches. This is stuff that'll be fixed with it comes out of Beta, and with other updates on the server side. Like the ability to set reminders for when I connect to a certain wifi network(instead of using a location).
General:
I don't hate the charging sound, but I do want to change it. It sounds just like when you turned on Tell Me in 8, and that bugs the crap out of me.
Notifications in general bother me to an extent, but this isn't really an 8.1 problem. For instance, I receive two emails. I open my email, read one, and return to my home screen. I'd like for my Email tile to still have a 1 by it since I didn't read the second one. I do this often with work emails. I'll get a BS email that I'll read and delete, then a work email that I want to save for when I get back to my computer cause its usually some kind of request to fix something or do a quick change up. The same thing goes for the Action Center, I'd like for the Action center to still show the notification at the top till I clear all notifications(I get a breaking news story that I don't feel like reading right this second, then forget about it).
I'd like the ability to turn off the Swipe keyboard or tweak the way it gets triggered. I've been doing a lot of mistypes cause it triggers on while I'm typing. Causing me to say stuff like "Look" instead of "lol"
All in all, there's nothing big about the update that I looked at and said "Well that sucks..." I'm pretty pleased about it, and am excited to see where it goes.