I was struck by a line in Seth's "Two Cents" article.  Seth was lauding
the virtues of a certain deck type for "decimating* newbies."  It got me
thinking about the decline of Harry Potter and added an additional
reason to the factors I was already considering.  I would like to
respond, not directly to Seth, but to the concept that was voiced, which
is unfortunately wide spread among all TCGs.
I think one reason the game has declined because people have gotten too
good at playing and no good at sportsmanship.  When Harry Potter
started, everyone was more or less equal in skill and card stock.  You
could go to league and not get creamed in every game.  Everyone won some
and lost some.  Now that some people have their decks that are the
equivalent of Pokemon's "Haymaker" decks, attendance at existing leagues
has dropped off.  No one wants to get beat all the time, less so if they
are new to the game.  Kids who get beat mostly likely won't come back,
won't buy more cards, won't help keep your retailer in business. 
Sheez, keep a starter deck on hand and when someone new shows up, play
them with that.  I can understand being concerned about the win during
tournaments, but during regular play who cares if you win or lose?  Was
it a good match?  Did you learn something new from your opponent?  Did
you have to try a new strategy?  Did you share a laugh?  Yes, you can
feel good after a loss as long as you played well.
Hopefully leagues everywhere will pick up after the summer.  Players can
volunteer to run the league at a local store, like I did.  Tell all your
friends, give out promos to new kids and the kids who brought them in. 
Run DCI tournaments (we only need 4 players if I don't play, and 8 if I
do) and see how fast everyone climbs in the rankings because there are
so few players!
And while we're at it, lobby WotC for better card art.  I'm embarrassed
to own some of them!
local Head Witch
* Responding to the connotation of decimate as meaning excessive
destruction; it actually means to destroy 1 out of 10, which I don't
think most newbies would mind in the least.  ;-)
  /\/\    Beth Johnson
 / o o\   Cosmic Wonderer
( / ^^\)  Springfield, MA USA
 \ M_M/   "Ruling a country is like cooking a small fish--
           you have to handle it with care."--Lao Tzu