The first feature is snap to grid which is present in Dia. The idea of this feature is not very radical. The editing area has a grid on it. If snap to grid is turned on, whenever objects are moved, their corners and sizes will snap to the grid. So my box for example will be exactly 6 times 4 grid squares, and is strictly lined up with the grid. This makes two things easy: creating boxes with the same size is easy, and lining up multiple boxes is super easy. Oh and drawing straight lines is almost too easy. This feature's relative is snap to object but I don't find it nearly as useful. What it does? If objects are moved close to each other, they are snapped together. Snap to grid on the other hand should be available in every program where objects can be positioned. Every. Program.
Dia also reminded me of another feature I really like: customizable keyboard shortcuts. Dia doesn't have those, but all KDE applications do. I don't think there's any limit as to how far this should go. I would consider it optimal if every single action in the application could be given a shortcut. KDE apps don't manage this, but their customization is easy. Pick an action from a list, assign a shortcut. My only gripe with them is that some actions I'd really like to assign a shortcut to are not on that list. The biggest "why?" of all: only 12 tab shortcuts available for browsers. Not quite enough. A lot of heavier programs have macros (but they can't fix everything). But: customizable shortcuts - not that hard to program. So where are they?
While on the topic of customization, how about customizable menus. Much like shortcuts, except instead of assigning actions to keys, they are assigned to user-defined menus. This is actually a feature I haven't seen much in applications and can't really name one. I have seen it in some operating systems. Most notably, my current Nokia phone. The phone itself is horrible. Can't remember the model number, but it's their current low end. It has the worst keypad I've ever used. What it has, is a "go to" menu behind one button on the main screen. It's a menu for collecting shortcuts. I don't need to navigate the stupid menu system to get to the most important things.
Sure enough, these features are mostly what could be called "power user features", especially the customizations. Actually, both of these features are very often available in games. Especially PC games. Which is yet another reason for UI designers to go and play some games. The power of these features is largely understood, so nothing revolutionary here. What I just wanted to say is: use them more! Use them everywhere. They're some of the extra features you actually can't go wrong with.