We don't know everything that will be contained in patch 4.1, but between BlizzCon and more recent information, Blizzard has already outlined a number of the major features from the patch. Let's take a look at those features, shall we?
Hyjal daily quests
A new daily quest hub will be added to Hyjal, one of the two Cataclysm starting zones. This quest hub will actually progress the overall story of the zone. If you've quested through Hyjal, you know that players end up healing most of the damage done to the zone. Patch 4.1 will reflect that, and we'll be questing in a healed Hyjal. Perhaps some of those quests will actually take place in the Firelands, similar to the Joust quests or the quest to kill Nemesis? Time will tell.
The quests in Hyjal ended with the player pushing Ragnaros back into the Firelands, alongside Malfurion Stormrage and Hamuul Runetotem. Malfurion and Hamuul then discuss going to the leaders of the Horde and the Alliance to arrange closing off the Firelands completely. This raid zone will very likely be the climax of that story.
The final boss of the Firelands raid will be Ragnaros within the Sulfuron Keep, and we're likely to learn more of the reasoning behind Ragnaros's aggressive interest in Azeroth. The plan as of BlizzCon was to include a total of seven bosses. Perhaps more of our old friends from Molten Core? It would be a good opportunity for them to revisit the fetch mechanic from Golemagg that they were unable to properly implement back in vanilla WoW.
Abyssal Maw instance
Patch 4.1 will introduce another 5-man dungeon in Vashj'ir, this one completely underwater like the world zone that houses it. Back at BlizzCon, the developers tried to assuage the fears of the playerbase by being clear about trying to avoid the "Oculus effect," wherein quirky mechanics turned the dungeon into an annoyance.
This instance will have four or five bosses. Based on Alex Afrasiabi's recent comments, among these bosses may be an animal god, if not more than one.
More classic dungeon changes?
Before the launch of Cataclysm, the playerbase was grumbling about heroic Deadmines and Shadowfang Keep. While originally expected to be in at launch, it was later announced that these dungeons would be in patch 4.1 and not available at launch. That delay didn't actually occur. Both of the heroic classic dungeons made it in at launch. There may not be any of this in patch 4.1 after all, or we might see a different set of dungeons return.
Enhanced dungeon maps
At BlizzCon, the developers also discussed the planned addition (or extension) of dungeon maps. In addition to 2-D maps of the dungeons (many of which we have already), the basic UI will offer lore background for bosses, their basic abilities (though no mechanical or tactical explanation of the encounter), and the loot they drop.
This UI addition is unlikely to eliminate mods such as AtlasLoot, but it will be a great boon to players who try to remain addon-lite.
As of now, that's all that we know about patch 4.1. Do note, however, that almost all of this information dates back to the end of October, and patch 4.1 isn't even on the PTR yet. All of this information could change at any moment.
World of Warcraft: Cataclysm has destroyed Azeroth as we know it; nothing is the same! In WoW Insider's Guide to Cataclysm, you can find out everything you need to know about WoW's third expansion, from leveling up a new goblin or worgen to breaking news and strategies on endgame play.