Resize large picture to small picture always generate blur. Also, all pictures that are resized by WordPress are blur. A sharpen filter in media options should be a good idea (as the cms ZenPhoto).
See 2 pictures as exemple: http://goo.gl/F1b8B
1: original picture 1600px is resizing to 225px without sharpness filter = the picture is blur
2: resize original picture to 225px with a sharpness filter = best quality, no blur.
My idea: add a sharpness options in Settings > Media for all resized pictures (thumb, small, large BUT NOT for original).