I think that the poor visibility of this icon is also related to the fact that it is kind of lost at the end of lines of text. As you scan down the thread list, it is not easy to immediately pick out the ones with a tick. Also, I don't understand why the tick should disappear on mouseover (but, as I mentioned elsewhere, I don't like any content to change on mouseover). My personal preference would be for the icon to be at the start of the line, alongside the other icons (as it used to be?) A contrasting colour would also help.