They use the same #naming structure and can be searched in the same way
They can be promoted to sprints
They can be shared with everyone, a line, selected team members, or out to external people
They add to your innovation score (you get more points for Open Ideas)
Open Ideas are ideas that team members create outside of the parameters of a challenge whereas Challenge Ideas are ideas that have been created based on the parameters of a Challenge.
Ideas generated from Challenges are done so with the help of an overview, description of what success looks like, and what the strategic focuses are. This gives a lot more direction as to what ideas should look like than Open Ideas.
Challenge Ideas are also much higher level. The goal of a Challenge is to collect as many high level, strategically aligned ideas as possible in a set period of time. That is why Challenge Ideas are brief, with a 50-word maximum for the description.