Solid Game.
Another great expansion of the Shift series. A few new surprises and bunch of extra secret stuff thrown in for replay value. The only let down is, other than the new innovative expansive map design this game just seems to be a slightly altered version of the previous two games.
I would recommend straying away from the solid puzzler design and focus on more platforming elements. Running through a maze, collecting keys, and unlocking doors is getting dull. There's plenty of way to include more action oriented elements to the game. Look at the old Abe's Odyssey/Exodus for the PS1. That there was a great game that combined both puzzle and action elements.
I say give the Shift series a rest and work on a new project. You can always come back to it a year or so from now and revamp it with more content. But releasing one Shift game after another at this rate will bore your audience. I'm sure you can come up with a more exciting and larger scale game.
On a side note: No, I didn't down vote you when I couldn't find the reset button. =)