Desktop Grid - this is how it looks now
http://i.imgur.com/nLFuyNI.jpg?1

Present Windows -- All Desktops
http://i.imgur.com/b2XTZNq.jpg?1

mockup
http://i.imgur.com/ocSz1E2.jpg?1

I already reported a brainstorm idea and bug report as a wishlist.
[kwin] window titles and window icon on Desktop Grid
viewtopic.php?f=83&t=107494&hilit=desktop+grid
https://bugs.kde.org/show_bug.cgi?id=304628

But developer rejected this idea.
"Given the different use cases I do not see why Desktop Grid should get all the (distracting) features from Present Windows. While Desktop Grid allows to have the windows arranged like in Present Windows this is not primarily meant for switching windows, but for recognizing your desktop and allowing you to organize windows between the desktops.If you just want to activate a given window the Present Windows effect is just the better choice."

No. I don't think is a better choice. Present Windows -- All Desktops has this big icons and titles that allow to identify window (what application it is) easily but it doesn't tell me on which desktop specific window is. It only allows switching to that windows but the point is that I don't always want to do that. Sometimes I just want to regroup/relocate windows between desktops. For this Desktop Grid was invented but when there are to many windows open on one desktop in Desktop Grid view it becomes hard to recognize which window represent specific application or instance of the application. If I have for example three instances of Dolphin open and some more widows, it is really hard to recognize winch instance is the one I look for.

I want to use Desktop Grid for arrange windows but now I can't do and I don't use it just because of above described limitation.
The second thing is close buttons for windows in Desktop Grid. Now user have to click on window and switch to full view (I mean to certain desktop where window of the application is and then close it using its own close button).
Closing from Desktop Grid view would be much more convenient and this function is already introduced in other desktops environments that has similar functionality as Desktop Grid.

I believe this doesn’t require a lot of coding and this very little change can greatly improve work-flow, usability of Desktop Grid and user satisfaction.

My mockup isn't polished. I should have make it better. I mean my intention is the icons where bigger and window titles where bigger. The point is that when there are many windows open I recognize then mostly by icons and titles. Of course actual window miniature plays its role in recognition process too, but when they are to fuzzy icons are the first and the most important thing that my eyes notice.

I hope someone else can write a scrip for kwin to make this functionality available.

Martin Gräßlin has a pretty interesting blog post on that.

long story short: given the current codebase it's a lot of work, but thanks to QML it will be easier in Plasma 2.