My thoughts are that comments/discussion to clarify the actual nature/terms of the challenge are fine and actually necessary.
General comments such as 'oooh that will be difficult' or 'right up my street' I don't really have a for or against on other than, like Trena says - just make the thread longer without really adding anything.
Specific comments such as 'Ive got an idea using a.../at ... location' just give other people ideas. I'm happy to share my (limited) knowledge and (half baked) ideas with anyone who asks... but I'm keeping them close to my chest until the challenge closes & the votes are counted
but then by all means. JMHO