Solution for those people who can't stand to play anymore and want to quit:
Leave. Goodbye; nice knowing you. See you in a week

Like many have stated already, the old design fitted very well with the theme of Spiral Knights. Remembering old forum posts, one of the main highlights for some players was the feel that SK had compared to other MMOs. The old UI certainly looked more appealing for me, while being simple and easy to use, and the new UI does have that impact on SK's theme. SK's theme is one of its biggest advantages over some other MMOs, and it also helps provide some immersion. Of course, this is just the UI, which is a relatively small part of SK, but as with theme and immersion, even little details can help.

Now, I'm not saying that I completely dislike this update. The HP bar for monsters is a very useful addition, and yes, I'm sure that we'll eventually get used to the UI. Just like the shard bombs, monster nerfs, binding update etc., many still dislike the changes. Getting used to it is one thing; you could get used to having to live in an uncomfortable environment while still seeking change.

Regarding the title, I assumed that it was a call for forumers to have reasoned, polite discussions. Asking others to stay calm when most already are, telling others to get used to something that they find hard to and/or continuing to make big assumptions in the face of evidence is one thing, but ignoring reasoning against your view, directing or hinting rudeness at others or being immature is another.

With all updates, there will be players who like or dislike it. Why not share and discuss opinions instead of denying or attacking those that disagree with you? Accepting other opinions isn't the same as agreeing with them. While I hate pointing fingers, Rotflol has an example that is too good to miss: "But this thread is about stopping the rage and rant so Djawed you are on the wrong thread to be talking about the bad stuff of the update"
As I said before, I assumed the thread title was for civil discussions that pointed out both the good and bad parts to take place; I didn't expect outright denial of opinions or points of reasoning. Well, good thing that most of the posts here are about pointing out advantages and flaws of the update.
Since the title doesn't support rants:
TL;DR-Old UI felt better, update had its positives. Reasoning is better than assuming that others are the same and can/can't adapt or must conform to a view point. Everyone's different, after all.

That 'freedom of speech' excuse everyone uses to act as a jerk has a limit.
We live in society, we have to respect each others.

Else go find your own island and start your own country and be as free as you want.
If you want to be part of a community, you can be 'free', but still follow some rules as to not kill each others with our freedoms :P

Really? Noone can see how this UI is not at all compatible with existing SK style? Are you all really blind? Do you see the menus?! How ugly they are made?

I really can't believe the things im reading here.. You are either blind, or you don't appreciate the SK style at all. It used to be one of the most beautiful games!!! If you can't see that, stop playing right now.

Every little thing they added sucks, and it's not done as it should have been. But yea, we can get used to it. We got used to a broken game, now lets get used to broken and ugly UI.

NO MORE.

Theyll come back... They always come back.