The ENC28J60 is a "bare" ethernet board. You have to implement any networking protocols (TCP/IP, etc) in firmware on the Arduino, which is relatively challenging when you only have 32k of code space and 2k of RAM.
The Wiznet chips used on the official ethernet board include both ethernet AND some sort of embedded CPU that does IP, UDP, and TCP for you (mostly), leaving more room in the arduino for actual application. (The cpu buried inside the Wiznet chip is probably bigger and more powerful than the arduino cpu...)