The big ones already mentioned are A.I., Level Design, Creativity, and Quality Control. I think in some ways quality control has improved- someone mentioned actual game controls are more refined and standardised which I agree with, but for the most part, QA has hit absolute rock bottom, mostly because publishers no longer need to release finished products anymore.
By that token, I'd say Customer Satisfaction has progressed the least as well- some of it unwarranted, to be sure, but there is no denying that games are now much more expensive to make and focussed far more on the bottom dollar than true innovation, diversity or artistic integrity. Mostly. I recognise there are always exceptions, however when something like SimCity comes out a complete mess, with broken forced online issues, with the publisher outright lying about it needing to be online enabled, and nobody being surprised... that really says something about the state of the industry.