Sure enough, if you look only at goals for per game, it looks like the league has taken a dip. But Mirtle was on the right track back in March when he opined that fewer penalty calls were the likely culprit in the scoring decline. The table below outlines average goals-for per game across various situations, broken down by season:
The basic story here is that 5-4 and 5-3 scoring is down, but all three of the major scoring situations (5-5, 5-4, 5-3) are still well above pre-lockout levels, and the 5-5 figure is actually up about 5% over last season. While I don't have ready access to power-play time per game, I do have the numbers for the average number of power play opportunities per team across the last five years:
Remember how people were complaining during the 2005-6 season about the endless parade to the penalty box, and how all those power plays were disrupting the flow of the game? Sure enough, as players adjusted to the new era, we saw far fewer PP chances per game this year (a decline of about 17%), but that 4.85 figure still tops anything from 2002-2004. Combine this information with the previous table, and what you get is that 5-on-5 play is leading to more goals being scored, despite making up a smaller portion of the typical NHL game - the adjustments are indeed working! The only reason that fact didn't jump out last year was because of the abnormally high level of penalties taking away much of that 5-on-5 ice time.
Basically, as players have adapted to the new rules, they are avoiding some penalty calls, but still can't get away with what they could in years past. The meat of the game, the 5-on-5 action, is indeed resulting in greater goal-scoring. So don't worry about tweaking the rule book for now, just sit back and enjoy.
All data from NHL.com team-by-team statistics.