Most relevant differences for the average person / gamer:
1. An average of 6% performance improvement assuming same clockspeed.
2. Lower power consumption
3. Improved Intel HD 4000 or HD 2500 depending on the model. The Intel HD 4000 is about 35% more powerful than the Intel HD 3000 on average. Basically it is equivalent to a Radoen HD 5550 desktop graphics card.