Hi,
As an ex-visio user, I am still very much a novice with Dia, but what I've seen so far is great. There were, however, a couple of features in visio that I found really useful which I haven't yet found in Dia:
1. Ability to change the scale of the canvas - so, for example, if I wanted to make an architectural drawing, I might want the scale to be in meters instead of centimeters, or possibly in feet/inches instead of metric. And of course, depending on what I was drawing, I might want quite a small scale. So I'd like to be able to specify both the scale and the units.
2. A dimension line - architectural drawings frequently show dimensions - e.g.
|<----------- 9.3 mtrs ----------->| or |<----------- 30 feet ------------>|
In Visio you could grab this as an object, stretch it out between two points of interest, and it would show the length of the line, scaled to whatever the selected scale was. This was invaluable when you wanted to determine whether a space was big enough to accommodate e.g a dishwasher as well as a 500mm cupboard.
Regards,
John Cousens