Here's a summary of some of the more interesting 4.0.1 patch changes.

I'll keep updating the list as I learn more.

- All characters can toggle on & off all Minimap options. For example, you can see Mining, Fish, and Herb nodes all at once now. (Just turn each option on.)
- Hunters can also now track Beasts, Humanoids, Hidden, Elementals, Demons, etc all at once. It's pretty awesome.
- Characters can now collect XP from mining Herbs and Ores. It's a great way to level new characters especially now that you can see both node types.
- Discarded PVP gear can be sold or disenchanted.
- Several other items that previously couldn't be disenchanted can now be.
- Tailoring Specializations have been removed permanently.
- Leatherworking Specializations have been removed, however it's currently bugged and you can re-learn the specializations from the quest givers.
- Transmutation cooldowns reduced (significantly) to 4 hours.
- You can pickup Master Riding (310% mount speed) for 5000G. Don't bother trying to fly in Old World yet, that will only happen after Cataclysm.
- New scaling spells & skills have made leveling much faster and most low lever characters quite overpowered. Health and mana regeneration is significantly increased as well.
- Older (Level 70) Heroic dungeons have been tweaked, making mobs hitting a lot harder. I used to solo Kara quite easily, but it's become a lot more difficult now.
- NPC that have quests available (or are part of active quest chains) now show text above their heads identifying them. It makes it much easier to find "points of interest". This was actually a very subtle, but extremely useful update and I liked it a lot.
- I like the new Thorns spell. Instead of a buff, it's an instant cast defense spell that tears apart melee characters. Works great on Rogues. =]
- Lockpicking now scales with your level. All 80 Rogues automatically have 400 LP skill. Since all skills have been removed, weapon training is no longer required and everyone always has full weapon training. Goodbye "Knuckle Sandwich" Achievement.
- Many Hunter Pets now have unique abilities that can benefit the entire raid. Right now, you can swap in-and-out 4 different pets depending on your needs. The 5th pet will be available when Cataclysm is released. Low level pets also now auto-level to 3 levels below you instead of 5.
- Level 20's can now move at 100% mount speed. All mounts now scale with their riding skill.
- Most PVP Daily Quests now no longer provide Honor Points but rather gold. I think only the Venture Coin quests down by the shoreline provide Honor Points now - but it's very little and not worth the trouble.
- No more vellums, it's all just one new all-purpose Enchanting Vellum for armor/weapons at all levels. Makes things much easier.
- Power PC, Windows 98, and Windows 2000 users will no longer be able to play WoW.
- Flight paths are shortened, and don't make as many stops any more.
- There are a couple minor pre-Cataclyms events going on (other than the earthquakes). There are new conversations taking place in Thrall's chambers for example, and Emissaries should be arriving in cities soon for the new elemental invasion quests.
- Honor Points and Justice Points can be converted into each other, however it hasn't been implemented yet. I think it's pretty cool that you can PVP for points, and then exchange them for Raid PVE gear. Here's the original post from back in April.

We do plan to have a way to convert Honor points (PvP) into Justice points (PvE), and vice versa, at a loss. The conversions will be possible, but it won't be a 1:1 rate, and you'll have fewer points after the conversion process. We won't allow the higher tiers to be exchanged for each other, however.

- Auction Posts and Cancelling now have a secondary confirmation request. It's a big annoyance, and there were plenty of other options available if their reason was to prevent automated auction posting. The thing is - it's automated.. they just need to add an extra step to their automation. So, all this does is hurt legitimate users (who now need to spend a lot more time in the AH) and I'm pretty sure they weren't complaining about accidental auction posts being such a huge problem or that they requested these new features. "Hey Blizzard! Can you make it first ask 'Are you really sure?' than ask again 'Are you really really sure?'. That sounds like a great feature that everyone will love!"
- There are also rumors of Shaman Tanking being possible in the future (Plate armor upgrades too.) I think it would be pretty cool for a new tanking class. All of the tanking classes are MELEE, but what about a SPELL CASTING TANK? That would just be cool.