These graphs might further facilitate one of the goals I outlined here.
The colored lines might represent (ascending) common quality items, uncommon, rare, superbe. The horizontal axis would be indicative of character levels, and the vertical would be item levels. Acquiring gear on one curve or another might be tied to content and its difficulty. Assuming a standard 'endgame' model, the highest difficulty would commonly be attacked, yielding the highest gear quality.
The sloping of the lines are pure logarithms with vertical translation. Their shape isn't so important. The taper towards horizontal might happen slower, but the goal would always be to have the slope at endgame (here, levels capped at 50) tend toward 0.
Furthermore, things need not be so strict as to be held to any given curve. There can be some leeway, a band of acceptable values that buffer a curve on either side. As long as an item falls within this band, it is free to hold an item value above or below the given mean.
Here, the blue line might represent the desired item quality curve. The points might represent random, individual items belonging to the curve. Some will be precisely as intended, others might be slightly weaker or stronger, so long as they fit inside the acceptable band.
Couple this with an item decay rate directly proportional to specific item value, and the concept of inflation ceases to exist within MMORPGs.