The "or longer" is not really correct. The point is, the sweet spot length for the "equalization" done by the HDMI chips at either end is 2 meters or 6 feet. Most people already know that longer cables might cause problems -- although HDMI is actually pretty forgiving up to about 12-15 feet. But what seems to surprise folks is that SHORTER cables can also cause problems.
So whereas with audio interconnects, the rule is the shorter the better, with HDMI that's not the case. Ideally the cable between any two HDMI devices will be a single run (no daisy-chained cables, switches, splitters, adapters, wall-plates, etc. and 6 feet long. Obviously for devices that are close together, it is easy to allow for a neatly coiled, 6 foot cable. For devices that are farther apart it is wise to look for a cable that claims it is good for 1080p video at that longer length. For example Blue-Jeans cable sells two types of "best" HDMI cable, with one of them spec'ed specifically for longer runs.
But even as many folks get "lucky" with their long HDMI cables, just as many folks ALSO get "lucky" with short HDMI cables. The 6 foot length is best thought of as a guideline then, designed to maximize one's luck.
For medium long runs, Redmere technology HDMI cables seem to be getting lots of kudos. These are "active" cables with a chip in the source end (which means they are also directional -- you have to plug them in the right way around for them to work right). Basically that chip handles the "equalization" in a way that's specific for the length of that very cable. To the HDMI transmitter/receiver chips in the devices at either end, it makes a Redmere cable of whatever length appear to work as well as a 6 foot long, normal, HDMI cable.
For really long runs (more than 30 feet) odds are you will need to run something OTHER THAN an HDMI cable. I.e., HDMI to a converter box, then something else between that and another converter box at the other end, and then HDMI from that 2nd converter to the destination device. There are several choices of "something else" and the matching converters, with a surprisingly large range of prices considering they all do basically the same thing, which is get you off of the simplistic, twisted-pair wiring that's inside an HDMI cable.
Edited by Bob Pariseau - 9/21/13 at 7:43pm