I mean that changing weight to bold is only one thing. What about changing contrast, width (without losing stroke integrity) and many other aspects.
The issue is complex and very interesting (to me at least), I see it in an evolutionary perspective, and evolution is not linear, it has leaps. One of the major changes (not to say advantages because this is a relative term) is the visible real-time modification, which is very different than "executing a command" and waiting to the result, and it has a significant impact on the design process. Of course it is far from the traditional design process, which is great and will always be great, but, admit it or not, doesn't suits naturally to computer technology. (Drawing by hand and using besier curves has nothing in common for example). Take Spacing for instance, You work today with classes, that's effective in a way. But i'm a designer in nature, when I see tables with numbers I freak out. With Fontark I simply and intuitively mark the glyphs I want to space (saves them as a group by a click) and tweak it up. It is like piano playing in opposed to filling reports.
So there's the traditional process, digital technology, and in between there's mostly human creativity.
I have tried Prototypo and was disappointed too, the advantages of tweaking a fixed typeface (coded in advance) are minor, and useful in a very narrow span of modifications, provided they're done well, and since you have to code every new typeface, this is just shifting the problem to another part of the process if not creating a new one.
Fontark's approach is very different, and I think it combines better the human-machine ways of work and capabilities, so to say. Prototypo is enlarging the distance between the designer and the typeface (by limiting the control to a set of sliders), Fontark let you deep both of your hands into it, like in an open heart surgery.
Another important and interesting aspect is that both PT and FA are private and independent initiations of designers (in opposed to code developers), with very little to no support at all in our ever growing corporational world, that can evolve and grow up to be really great, but as said, community's help is fundamental to it's success.
Fontark represent a different approach to the same issue...
Fontark's first and leading concept is "Never limit the designer". This concept led to a very flexible platform where the designer can create any desired connectivity between the font glyphs and have full control over any aspect of the design.
Fontark's meta font is a Skeleton. - A Skeleton is a line, which is much easier to control than an outline and much faster to draw... and "code", without coding at all. "Skeleton based" type design is not very fond over type designers, for understood reasons, but we see it as the basis for a much complex and flexible control over a non-consistent outline in the near future.
From my experience with it it's gives a completely new type design experience, controlling and operating it (once the basic principles has understood) is so fast and smooth it put's aside so much of the technical aspect of drawing the font and confronts you right away with the core of the design itself.