Writing about the Patch 4.01 changes also made me think about the impact of the changes coming in Patch 4.0.3 and the Cataclysm expansion. While the current patch requires us to learn new shot/spell rotations and make some other adjustments, I think that the changes coming in the December patch and the Cataclysm expansion will actually have a greater impact on our game play.
Flying in the Old World, of course, will be awesome, and it will be fun to explore the new zones and try out the new races, or roll a toon in one of the new race/class combinations.
But the elimination of almost all existing quests (in the Old World, at least) in favor of new ones is a major change, and I am pretty sure it will take time for guides and databases to catch up. This is true of add-ons like Gatherer, QH2 and Carbonite, which depend on data collected from players over time, as well as leveling guides, which will have to be rewritten from scratch. Auction house scan data and knowledge we've collected over time will likely be impacted too.
We and our information resources will have to adapt to, among other things, the recalibration of most zone levels and thereby the level flows. Much of the reference material about quests, spells, abilities, geography, travel, vendors, items, gear - just about every aspect of the game - in sites like WoWHead, WoWWiki, and ThottBot will become obsolete, and it will take time to build up new and accurate information repositories. In the meantime we're going to be fumbling a bit.
We're going to have to learn our way around the world again. The loss of the capitol city portals in Dalaran and Shattrath is just the tip of the iceberg; so much of the land will be changing, zones reconfigured. What we know about sneaking a low-level Alliance toon past the Horde mobs at the Mor'Shan Rampart to get to Ratchet and thence Booty Bay, for example, or through Dustwallow to Theramore or Wetlands to Menethil to access these vital intercontinental harbors, or running a young Horde down from Arathi Highlands through Wetlands and Dun Morogh to Badlands or from Ghostlands to Light's Hope or past Darkshire to the Swamp of Sorrows for those key flight points - most or all of this knowledge may well become useless.
And there will be so many new flight points! On the maps it looks like double or triple the number. Travel for low level toons will be more convenient, but also more confusing at first. While leveling, we're going to have to remember to look for flight masters everywhere we go.
I think it was wise of Blizzard to split the changes into two parts, so we have time to adjust to the character-related changes before we have to learn about all the world changes. Cataclysm is going to be almost a whole new game. Bigger, and - I hope - a lot better too.