WotC are very good at marketing... Magic. So why not try some of their tricks?
Examples:
Weeks after the initial announcement they could have had polls up where fans get to vote to save their creature - that way getting a sneak peek at what will get into the MM, and also feel that the designers actually listened to them.
Later on they could have had similar polls asking which classes and races that aren't making the first PH are most important to get to see as soon as possible.
And right about now they could ask us which so far unused power sources we are most interested in, and even hold a contest as for inventing a new power source, including a few classes and powers to show off what it would be about.
For more simple audience participation - why not "make your own power/feat/monster"-competitions where they would show a few samples just to whet our appetites.
As for things not to do:

'In 3rd edition this thing X is just so bad. Don't you all hate that? *laugh* That is why we made X into a Y now and it is much sexier.' (aggravates X-lovers and feels cheap to many others)

'Ok X has been in the game from the beginning. But nobody really understood it anyway so we are taking it out of the game.'
(makes them look stupid to all those who feel that X is something that they understood and loved about the game)

Publish preview adverts in the form of books and ask people to pay for them.

Remake the website into something uglier and more confusing where visitors have to work to see what has actually updated the last day.

Promise that things are going to be ready by a certain date, and then not deliver.

Allow information that is no longer true to be accessible from the website.
I think 4th edition is really good. I think that some of the marketing of it is really good too. And I am really glad that it is selling so well so far despite all the bullets they have put into their own feet.