Very surprised to see no one mentioning yeD here. It is _the one_ diagram making tool I go to, when I make any diagrams related to software development. The only areas where I had issues with it, that I can remember were: - "How do I teach it to make these halfcircle-circle joints of component diagrams?" - Sequence diagrams are a bit slow to make with it, because yeD not knowing their semantics and not doing much for…
yEd is really great! I stumbled upon it while googling about 7 years ago, and it was awesome even then (and it comfortably beat visio). What's even more cool is their (newish) online version, which means you can do all the diagramming in the browser and even share it conveniently. Thank you to the creators of yEd - you've put together a wonderful product! If there is one feature I would like to see, it's the ability…
As for your wish, I think that's already there. You can select many items at once and change them all in the same way, e.g. changing the label font. To select all elements of the same type, simply select one of them (e.g. a label (you may have to use Ctrl+click to select a label on top of a node)) and press Ctrl+A. That should select all labels, while not selecting anything else. From there you can change properties for all of them at once. I think we've added this to yEd Live (the online version) as well recently.
There is also a powerful selection dialog under Tools > Select elements where you can basically select everything that matches certain criteria, such as all nodes with the same color, or predecessors of the currently selected node. Afterwards you can change properties in the panel as usual. This one only exists in the desktop version, though.