I don't know how you're going to get personal taste divorced from the voting. The Challenges Help article here at DPReview says we are to rate each image according to our own interpretation of how aesthetically pleasing the image is and how well the image meets our interpretation of the challenge's criteria (name, description & rules). Each person's biases about the nature and purpose of photography and the likes and dislikes we've developed over time will determine how aesthetically pleasing a particular photo, or even approach to photography, is to us.
Once I began to consciously evaluate images for clarity (in proper focus), composition, creativity and degree in meeting challenge criteria - then it became fairly easy to recognize the remaining 'like it' value is just my personal taste. Instead, I've added another factor of 'uniqueness' - or how much is this
not like any other picture of this subject I've seen - whether or not I like the subject.
Additionally, if the challenge has absolutely no rules or if the host has not DQ'd an image for violating their rules - then I will not vote 0.5 unless the entry is of horrifically poor quality and bears no resemblance to even the title of the challenge - so, usually not at all. (At least that's how I'm voting now - historically, I got a little too literal and voted a lot of 0.5s.)
Not sure why, but I did not vote in the 50 series. In the 25, I only voted for 4 that I felt earned 4 or 5 stars. If you interested, here's my 36th place in that challenge >
http://www.dpreview.com/challenges/Entry.aspx?ID=507382&View=Results&Rows=25
(Guess I should have cropped it some - to get past the thumbnail vote!! lol)