System Information
Windows 7 64 bit
AMD Radeon HD 6970

Blender Version
Broken: Blender 2.70 and 2.71
Worked: (optional) Blender 2.69 and earlier

Short description of error
When using the merge menu Blender auto selects the last used location from when you merged last time, in earlier versions it chose methods based on how you had selected elements earlier, now it always goes with the last used location no matter what. It's not easy to properly explain what's happening, but see the steps listed below for a practical example.

I'm not sure if this is actually a bug or not (might be an intentional design choice) but I thought it worked a lot better before. I liked the older method a lot more as I very often merge at the last element, but sometimes need to select a whole loop and merge it at center, in earlier versions that never made me stop and adjust back which now lowers my modelling speed and makes Blender feel less streamlined.

Exact steps for others to reproduce the error
Steps to reproduce:

1. Add a default cube.
2. Select two vertices (with Shift+RMB)
3. Merge these two at the last element and deselect them.
4. Select two other vertices (with C [circle select] in one go).
5. Merge these two at the center and deselect them.
6. Select two other vertices again (with Shift+RMB).
7. Try to merge these vertices and you'll see that Blender thinks you want to merge them at the center, even though you selected at last the last time you could.