So what makes an idea "Hot"? A hot idea is one that has lots of activity in a short period of time - both votes and comments.
Can I change the default sort option for my forum? Yes, click the icon in the bottom left corner-> General -> Feedback Forums -> Scroll down, and you'll see the option to update "Hot" to either "Top" or "New" for your forum.
I'm worried users will miss good ideas: This is one reason we recommend SmartVote polls which allow you to proactively get feedback from your users, and have them rank ideas you're considering. This article goes into more detail.