Hi guys,
The 'big four' (Fighter, Rogue, Cleric & Wizard) are coming out first, for obvious reasons of popularity and it was always our intention to see how well they went before committing ourselves fully to this line. However, it is fairly clear that there is an extremely strong demand for these books (we upped the print run of Fighter and it still sold out in less than a month - the reprint is being done this week) far beyond that which we expected, hence our announcement on starting the race-based Quintessential books.
Overall, I would be very keen to see every class and race detailed eventually (there is nothing quite like having the entire set of something), but it kinda depends on you guys

So long as the Quintessential line remains popular, more books will be released, though I imagine the 'big four' will always be the best sellers in this series. That said though, we are also planning a few 'special' books, the first of which will be out around September, that will broaden the horizons of the series somewhat, thus keeping it fresh with material that has not been seen up to now.
The one thing I want to do is make every class and race actually feel different - the main target with this is the Wizard and Sorcerers books. I have a feeling many players think them a little too similar to each other in the PHB, and so we will look to put a crowbar inbetween them and make them a little more distinct - the trick, of course, is to do this without 'breaking' any of the current rules, but we have a few ideas. . .
We have pooled our main writing talents for these books to ensure each is the best that it can possibly be, rather than degenerating in an ever-decreasing spiral that another, very similar, series started to do at the end of its cycle. So, Sam Witt (Chaos Magic) has completed Cleric, already started on the second race-based book (announced next month) and has requested a third (!). Mike Mearls did Rogue, due out in a week or so, and is already tearing through Wizard. Alejandro Melchor (Constructs, due out early March, watch out for it, he did a superb job!) is doing The Quintessential Elf and has expressed a (very) strong interest in making Bards everything they can be. So, in terms of actual production, there will be no shortage of new ideas.
Hope that answers your question!