Injuries are the biggest factor, but those are something you can't predict so it's not something you can really take into consideration unless you're looking at individual players with dodgy injury history
I think the Broncos will go close to the minor premiership. Bennett sides always like to start fast, especially the Broncos who know they'll have a testing time around Origin. However, even that issue has been improved for them slightly since Bennett was last with the club.
Whether fans/those in charge admit it or not, the almost Friday fixed schedule helps, especially early in the season when it's more consistent.
It's for these reasons along with their very good side that I think they'll go close to the minor premiership.
However, I have the Roosters as my favourites all things being equal. Despite the loss of Maloney/RTS I think their forward pack is about to go a further level up. Last year they had guys at their peak (JWH, Friend, Moa, Cordner etc) along with guys who were making an impact but still a little raw in some ways (Taukeiaho, Evans, Nappa etc). I think this year they've got those core players still at their peak, but they also have those younger guys ready to move up to the next level. If they stay fit and focused I can't see too many packs sticking with them for 80 minutes.
The Cowboys I had a feeling around round's 7-8 that they were just destined to win the premiership this year. That just doesn't happen two years in a row and as a result I never tip the reigning champion.
So I really don't think theres much wrong with so many tipping Brisbane, but injuries can change everything in the space of a round or two.