Depends on the nature of the contest. If it's a smaller/simpler contest, the theme would dictate my interest. If it's something more ambitious that measures your skill, I might be more interested.
Back when mocpages still had an active community, there was an annual contest called Bio-Cup, which pitted bio-mocers against each other in varying themes and increasing challenges. I loved that contest, as it served as a means of challenging your comfort zone and measuring your skill within the community, and that in itself was quite attractive about it. At best it had 84 contestants and lasted 3 months, but it was totally worth the attention and bragging rights. It never had physical prizes.