Support for a 9-slice button would be a great addition indeed. I just want to add that Minetest should not use a heavily-stylized default button design. IMO Minetest's default button should not look like Voxus or any particular subgame. Simple, but still “inviting”, as you said. I have nothing more to add on that.
I am happy that finally someone has successfully defended against the “Minecraft argument”, that is the accusation that you just want to make Minetest like Minecraft. Dudes, it's not always “because Minecraft”, but more like “because it is a good idea and
coincidentally, Minecraft has done that already”.
Now for bitmap fonts:
Using a bitmap font would be no justification to break the rules of English grammar. ;-)
One concern I have about bitmap fonts is that they often only support a tiny character set. Especially very detailed scripts like Chinese characters can't be properly displayed if the available area is too small.
Bitmap fonts which cover a large character set are rare.
But optional
support for bitmap fonts would probably not hurt Minetest.
If I understood correctly, this suggestion is not actually about bitmap fonts vs TTFs, its about “pixel” fonts versus sans-serif fonts.
GNU Unifont is a great quest to create a huge bitmap font (also available in TTF) which covers as much of Unicode as possible. Sad but true, but the creator(s) of this font said that it seems the smallest amount of pixels where they could possibly fit in Chinese characters is 16×16. Everything smaller than that would simply have far too few detail.
Another problem: Some of you argued that the player could change the font on his/her own. My question is: Why should a player be motivated to do so? Probably only because of a poor default font in the first place. IMO it should
not be the task for the gamer to make the GUI design. I think typography is totally part of the GUI design. So choosing a good default font is crucial. Also, a lot of mods rely on the exact size of the current default font, they'll probably break if the user manually changes the font. That's because Minetest does not handle custom fonts very well and mods have 0 idea with what actual font they are dealing with.
I am not saying the player should not be allowed to change the font, but the default font should be already a high-quality font. I am for sane defaults everywhere. :-)
On the other hand, maybe it would be an even nicer (but probably a bit difficult) idea if Minetest supplies a sane default set of fonts. At least one sans-serif font, and at least one “pixel” font. Ideally both fonts already cover a large character set and are interchangable (means, GUIs don't break if another font is used in place of one font).
It would be also cool for mods to change the font on the fly, but I guess this change would be way to huge for now. I suppose mods sometimes would want a sans-serif font in some occasions.
Another problem are formspec definitions. They are a bit complicated and have an arcane syntax, but I can actually live with that, it's not too hard to learn. IMO one of the bigger problems in formspecs is the fact that coordinates are inconsistent.
Most widgets use inventory slots, but it seems that some widgets use a coordinate origin which is displaced from other widgets' origins. Other widgets don't seem to base on inventory slots at all. Nothing of this coordinate mess is documented and it is very confusing.
Another problem is, while you could theoretically resize the HUD elements, a lot of mods (including mine) have huge problem with HUD elements in a size other than the default. You will very easily run into overlapping problems. I have still not found a clean way to handle HUD resizing from the mod side.