Patch 4.0.1 is a prelude to Cataclysm. The patch contains most of the basic mechanics and ruleset changes for Cataclysm. These are mechanics changes such as the redefinition of spirit, which is now a healer-only stat. Guild reputations are in place, even though there's no way to actually generate that reputation.
This buffer period gives Blizzard developers time to adjust the servers and various back-end systems. The gears can get oiled up real well, and the developers can handle issues before they wreck the launch of Cataclysm.
What's the biggest change?
The biggest change that affects your daily playstyle is going to be the large-scope changes to talents. You no longer simply spec a little bit here, a little bit there. Now, when you choose your first talent point, that will determine what specialization you are within your tree. Why is that a big deal?
This is actually a great blessing to those of us who enjoy the casual or rookie mindset. There's a certain amount of safeguarding here for those of us who don't want to number-crunch the very elitist of numbers. By restricting our talents within a certain tree and giving us our main abilities early within a tree, Blizzard is helping to ensure that we spend our points in a viable way.
Once you've earned the most advanced talent in your main tree, your other two trees will become active. You can choose to take a few talents in secondary or even tertiary talent trees, but this still won't require the high degree of precision that pre-4.0.1 builds required.
What else is huge?
Your addons may need updating. Enough gems have changed colors that you might need to swap them around a bit regain your armor's set bonuses. Your glyphs will definitely need updating because you can now learn all glyphs and basically decide which ones are active at any given time.
Where did my badges go?
OK, here's what happened to your badges. Badges began to be used in The Burning Crusade. The idea at the time was to provide supplemental gear for folks whose items didn't drop during raids. Folks could take their badge gear to fill out their armor kit and move on to the next raiding tier.
Since that time, however, there've only been about a half billion new raids, and of those half billion, a quarter billion have had heroic modes. And then a third billion have existed as both 10-man and 25-man raids. And every time we've stepped up a tier of gear among these different raids, we've needed a new type of badges in order to differentiate which gear to buy.
It ultimately ended up being a huge pain in the butt. So in Cataclysm, Blizzard has simplified the whole scheme. There are basically four different types of purchasable gear in Cataclysm, so there are only four different types of points to buy it with. At the end of the day, this all means that Blizzard had to convert extant badges into the new schema. Some of the oldest badges even got changed into gold instead of points.
This is confusing!
First, don't panic. Check out our information roundup if you want to read about the changes for your class.
Second, remember this isn't the first time we've all been through this. We had similar transitions between vanilla World of Warcraft into The Burning Crusade and then again from BC into Wrath of the Lich King. These few months can be challenging but will allow us to flow into a better, more balanced Cataclysm.
Things will feel way out of whack for a little while. DPS numbers change, health changes, healers feel different. It all adds up into feeling like virtually an entirely different game. But when we all launch into Cataclysm and get to rock out in the new expansion, it will feel worth it.
Visit the WoW Rookie Guide for links to everything you need to get started as a new player, from how to control your character and camera angles when you're just starting out, to pulling together enough cash for mid-level expenses such as mounts and dual specialization, to what to do when you finally hit level 80.