A patch? I don't think so... should of it been part of the original game? yes of course. For some reason, I feel that I wouldn't appreciate the game as much if It was not an expansion pack. I think it is well worth your money. 2 new character classes, 800x600 resolution (MUST have if your playing on a 19" or larger screen)(They should of included even higher resolutions like BG2), new items, new act. for $30-$35 it was a bit steep, but the improvements in my opinion are substantial.
The rankings now have an expansion set ranking. Expansion players can go on that instead. Problem is, you can convert a character to an expansion rules character thus throwing you immediately into the expansion ladder rankings. The only way I can see a resetting of the ladder to work is to erase everything and start over.
My favorite features would have to be the better socketting system, the new resolution, and the larger stash.
Those things above I think add more to the game than the new characters. It is a tad buggy, but the only problems I have had is with battlenet and not the game.
I have not played Act 5 yet so I cannot give any input on the quality of the expansion story.
Hope this helps in your decision.
btw... once the price goes down to $20, it is a MUST buy. Don't knock the expansion... remember hellfire? It could have been that bad.