1) homerun animations: My biggest pet peeve is when you hit a homerun and you can never tell where the ball is going. I play as the Jays (im canadian so duhh) and on most games when I hit a homerun, I can never tell where I am hitting the ball and where it is going. I want a game that clearly shows you cranked the ball into the 5th deck out in left field. As horrible as the Triple Play Series was, I used to love how the ball would bounce off the upper decks and land back on the field.
2) stadium animations. I heard that WSB has great stadium animations. It's always annoying when you look at a stadium and nothing is going on in the background.
3) the other little touches. All Star Baseball had the neat little touch of seeing the pitchers warming up behind the outfield walls (not sure if the WSB had this last year) and the signs looking identical to the ones in the real life parks....I was always a big fan of the small rain delays and weather animation.
All those examples may seem like extremely small details, but for some people like myself, it can make the difference between just a rent, or a purchase.