Patch 4.0.1 is the second to last major patch before the Cataclysm hits - and it's definitely major!
In this patch, many aspects of the game are drastically changing to adopt the new systems which will be used in the Cataclysm expansion world., However the world itself will not be changing - not yet, at least. Blizzard states that, that will not happened before patch 4.0.3
Here are some of the changes made:
· Classes: revamped talent trees, updated hunter pet system, new druid, hunter, paladin, and warlock resources, new Mastery stat
· User Interface: improved Spellbook, new Guild display options, updated PvP interface, expanded and enhanced Professions tab
· Systems: glyph updates including all-new Prime glyphs, new Reforging ability to customize item stats, new PvP and PvE currency system
· And much more!
This is directly from an Bluepost from wow-forums, which means that it’s a Blizzard employee posting.
Resources: wow-europe and wowhead.