So, what is the difference between the S&P 500 at around 1,520 now than in those "bubble" years of March 2000?

The key difference between today and March 2000 is the nature and valuation of the sectors leading the charge in each case. Seven years ago, the sector leading the charge was technology and, in particular, the Internet industry, which was really in its infancy. These leading sectors had valuations at that time that were wildly out of control. Tech sported a 12-month forward price/earnings multiple of 48X back then, which was nuts.

Leaders today are so much different. No single industry has dominated the recent move back to the highs, but financials and energy are at the front of the line. Valuations for both groups are entirely reasonable, and probably even low, at just 12X. The forward PE for the entire S&P 500 today is a mere 15.4X, which is close to the index's long-time median.

Based on this valuation data, and reasonable projections of growth, it's quite clear that the market is anything but frothy now, or overbought. The financials, energy producers and electric utilities should be able to maintain their leadership role and will probably be helped by health care. To keep the momentum moving in the right direction, all we really need to see is U.S. core inflation right around current levels, or edging lower, as recent reports have shown it to be doing.