Title says it all!
For example, I would like to be able to see (in an extra column) a red or a green arrow showing a fall or a raise in the price of a card (seen it in a few other sites) - at least for some of the staples, if not for all cards.
Another thing i would find awesome, would be to be able to track the value of some selected, (or all your collection's) worth through time (displayed in a time/value graph). (to be able to see depreciation etc. of your cards).
I would love to see even more from this site. Its totally awesome even now and i use it every day.
P.s. Sorry for the low quality of my english (its not my first language , its morning here and i couldnt make a good sentence to save my life right now 
).