Considering, with the exception of the year-round parks, most attractions debut more or less when the park opens for the season in early to late Spring. The window to experience and vote is usually very short for new attractions at regional parks. I realize you're also helping advertise new attractions and that's good for everyone - just seems the eligibility period versus the average debut date is skewed to the Big 10 parks.
A good example might be "The Voyage" at Holiday World. It's been named the best wood coaster by Amusement Today for 4 years in a row. It debuted on May 6, 2006 giving it about 7 weeks to earn enough votes. The winner that year (Expedition Everest) was opened to annual passholders and DVC members in January (I know because I was there and couldn't ride it - so I never got to see a working Yeti) - giving it much longer to amass votes. (Grated EE probably would have won anyhow - but think how much a boost this might have brought Holiday World if The Voyage had been given an even period of time to be experienced and rated and won.)
Just my two cents. I know you didn't ask...
Harry Potter opened in mid-June last year and won in a walk.
Most parks - even Disney - now try to debut their new rides around the Memorial Day weekend, if they don't open for the start of the season in March/April/May. Rarely do new rides open in the latter half of the year anymore. It would be those rides that would be at a disadvantage in this system, honestly, since in our experience, average ratings tend to diminish over time and as more and more votes are cast. A ride that debuts in June to a blast of early favorable reviews has the best shot at this reward.
That said, with so many rides debuting around the same time now, they're pretty much on an even footing to start. The public's reaction to them will determine the winner, as it should be.
Looking back, I think that the award's done a pretty good job of selecting the best attraction of its year. The Simpsons Ride over Toy Story Midway Mania might be one debate (and that was a razor-thin margin). And I still don't understand how Mystery Mine beat Maverick and Griffon. But the rest are solid.
Even Everest. Remember, at that time the Yeti worked. While Everest isn't as a great a coaster as Voyage, with a fully functional Yeti, I'd argue that Everest is a superior attraction.