Well in comparing CVS to CVS2,i can start with the graphics.
First of all the character sprites seem more pixelated in CVS2 than the original(i have know idea why though),also half of the returning 3-D real time effects are more pixelated as well.
The reason behind that i know,Capcom chose to scale and enlarge the effects to make them look bigger and huge.
In the process though they became pixelated.
Some of the new 3-D real time effects are much better than the previous ones.Most noticeable one's are Ryu and Ken's fireballs.
But Kyo,Iori and Mai flames looked so much better in the original.
Now as for the backgrounds it all comes down to what you prefer,2-D or 3-D.
To be totally honest both games have beautiful backgrounds.I like both titles backgrounds equally.
The original Capcom vs Snk backgrounds surpasses even the Guilty Gear X titles backgrounds(most so in style and overall artistic quality).In Capcom vs Snk 2 the backgrounds are 3-D but they're done in a way to mimic the artistic quality of 2-D and they are simply beautiful.They also are much more detailed and better designed than the backgrounds in Marvel vs Capcom 2(i do miss the Snk style intro's from the original though).
Now on the matter of sound,the sound effects and voices are pretty much even with each other.The soundtrack on the other hand is much better in the original(although i would have prefered character theme music over the techno style music used in CVS).
But CVS2 have a few awesome tracks,most noticeable ones are,The London Stage,Shin Akuma theme and God Rugal theme.
Infact i would go so far as saying that Shin Akuma theme is the best track out of all the tracks thats in both versions CVS and CVS2.Its really that great!
Now gameplay is where CVS2 really outshines the original CVS.It has so much more than the original,6 button,6 grooves,EX customize style grooves,grooves are more complete unlike the original where the only thing that seperated them were the way you perform your supers.
Much balanced gameplay,free ratio select,larger cast of characters and so much more.
In the end though CVS2 is a much better game than the original CVS.It excels over it in the most important area and thats gameplay.