HL2 EP3 was sorely missing from E3. Instead, this was unveiled.

As for the whole "Why want to release a sequel when DLC could do it..." - - I don't think Valve ever expected fans to react in such a way. In a way, the fans are right.

Watching the gameplay videos, I must say it plays just like L4D, with improved weapons and chainsaw. Perhaps its Valve's move to introduce L4D for those who have not played the game before? Thus far the response to a 'sequel' has been very mixed, mostly questioning why it has to be a full fledged standalone sequel.