You've already heard that the Amani War Bear is disappearing from the live servers ahead of schedule in 3.0, but here's one more thing for you to hurry up on: 40-man Naxxramas. As of now on the 3.0 PTR, Naxxramas is gone from Eastern Plaguelands.
The whole of the Eastern Plaguelands has already changed, in fact. When you open your map, you can already see Archerus looming ominously over Light's Hope chapel and the remains of the Scarlet Crusade's holdings in Eastern Plaguelands. As a result, the entire map has shifted, but I still combed the Plaguewoods looking for the entrance to Naxxramas.
Due to the map shifting, the old locations for Naxxramas are rather off. At the original location coordinates for Naxxramas stands one of the old ruined villages that dot the area. The area where Naxxramas used to be relative to that Scourge ziggurat on the map is now marked by a plague cauldron. Regardless, It looks like the entrance is gone. Come 3.0, it looks like Naxxramas will have retreated to Northrend, and we'll have to bide our time before we can follow it.
So if you've been waiting on getting those last few splinters, or you've been thinking about picking up a Corrupted Ashbringer or what have you, you'd better get on it fast. When the 3.0 leaves the PTR and goes to live servers, 40-man Naxxramas will be gone indefinitely.