Subtitles can be useful if you're in a competitive category and you want to set yourself apart from your competition with an offer like "Free bonus widget with every order", or "Free shipping with every XX ordered", or "includes xx day warranty". The goal is to include a statement that will get your item clicked ahead of your competitors.
I often see sellers not using the subtitles feature very efficiently, loading them up with keywords not realizing that the words won't get caught in a search.
One thing I don't see very often is the subtitle used to insert humourous statements, like "My wife will kill me for selling this!!!" or "Need beer money... buy meeeee". I think those would probably generate some clicks although I'm not certain how serious the browsers who click would be.
I'm sure the Bold feature gets some additional notice, but again it depends on how much competition your category has and whether or not the fee is a significant portion of the profit of the item.
I've seen some categories where bold was so common, the listings WITHOUT bold actually stood out more, lol.