If you float multiple elements & one of the div has larger height then others, then these types of effect are created (the one you showed in your screenshot).
Solution 1: clear float left from 1st element of each row using :nth-child(2n+1)
in your case its ..2n..
cuz you have 2 elements in one row.
Add this css in your style-sheet:
.thread-list:nth-child(2n+1){
clear:left;
}
Solution 2: Solution 1 will align all the div's but there will still be a negative space beneath, if you dont want that then you have to use plugins like Masonry Layout, this effect can not be achieved with pure css.