Class Combinations Breaking the Lore
Recently, WoW players have the hue and cry about race/class combinations Blizzard hasn't selected gets louder, they complain that it is bad to their WoW Gold getting and WoW Power Leveling. Blizzard reiterated that while the lore does evolve and changed, and some things that seemed unlikely before become likely, that doesn't not mean all situations that fit the bill do so. Game developers have a certain direction they are taking the story and the races, and currently the race/class combinations they have not selected so not fit into that story. Trust World of Warcraft! You experience of fight for World of Warcraft Gold will be better!
Argent Tournament and Cataclysm
It must be kept in mind that not every WoW player will be purchasing Cataclysm on day zero, so the content and stories of the previous expansion needs to keep for those players and any characters leveling up.
Argent Tournament allows you to gain reputation with city factions, then help you get WoW Money, so it will continue to be relevant. Some people still head to Netherwing Ledge and get their reputation/drakes because they want to do it on a new character.
Generally speaking, Blizzard doesn't like to spend a lot of time on new content and then make it end and disappear, they have to be very careful on when and how often they do it. Blizzard is changing some of the classic continent because they have been there for 5 years so developers feel it's okay to change it around after all this time.
The Onyxia example is a special circumstance for the 5 year anniversary. This isn't something Blizzard plans to do for every instance.
Old Instances in Cataclysm
Blizzard is not going to remove every instance which is what you seem to be getting at. If they do decide to remove anything they will let you know. Your game experience to get WoW US Gold will not be affected very much.
As far as items go, some may have their stats altered but this isn't set in stone. You can likely expect items that have strange combos like strength/spirit to be changed, but they'll have to wait and see how that all goes when they get to it.
Dalaran and Cataclysm
Blizzard doesn't plan to move Dalaran now. Without it, your WoW Powerleveling through the Northrend content would have a hard time.
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