Let's get real here, the thing that improved really amazingly here is merely sahding/coloring. The 2001 picture already shows a good grasp of composition and anatomy. All other changes over the years are pretty much detail over detail.
By the way, sayori's comment on this: 2000-2004: Mouse drawing. 2006, 2007: Line art done with a mechanical pen and coloring with a tablet. 2005, 2008, 2009: Tablet drawing + coloring.
TODODeygulash said: Let's get real here, the thing that improved really amazingly here is merely sahding/coloring. The 2001 picture already shows a good grasp of composition and anatomy. All other changes over the years are pretty much detail over detail.
I dunno, the gap between 2003 and 2005 shows a pretty big improvement in the characters' poses, and then the facial expressions really evolve from 2005 up to the present. It's hard to put your finger on because of the layers and layers of detail heaped on starting in 2007, but the characters in 2010 are much more natural than they are in the generic eroge-style CG of 2004.