Crimson

What you are looking for is the Alpha Mask Import plugin, I believe. It's relatively inflexible by design (it doesn't actually become a channel, it just applies a one-time effect to look like one) but should function exactly how you want it. Edit: not fast enough, it seems.

You do nothing in your post but compare it to Photoshop.

Image -> Canvas Size is what you are looking for. Simply set your canvas width and height to you preferred pixel ratio (1920x1080 or 1280x720, I'm guessing.)

Could you please explain what "fullbright" means? Maybe it's just me but I have absolutely no idea what that is, nor does Google reveal anything relevant.

By "unscratch," do you mean remove scratches and defects in a photo? You can use the Clone Stamp tool to cover up those areas.

Maybe it's not up for discussion, but I find the advertisements embedded into the words (especially in the menus selections) rather obtrusive when I'm trying to click a link and then inadvertently pass my cursor over one of the ads. Other than that, it's very intuitive.

Drop shadows are somewhat of an outdated design trend, and they only really work when executed EXTREMELY well. IMO I think you should leave the blu-ish halo as it is and make the last few letters a darker shade of gray to make more impact.

There is no way to easily adhere text to a complicated contour with Paint.NET. However, if you want to curve text into a circle, you can type it out normally then run the Rectangular to Polar effect. Look in the tutorial section to find out how to use that.

You can use the Clone Stamp tool to cover up the original text (this takes some practice and a certain degree of meticulousness). Then, on a new layer, type out the new text you want to insert.

Currently, Paint.NET does not have a built-in re-editable text function. When you finalize your type, you cannot go back and add/delete characters or change size; you must type the entire thing over again. There is, however, a Re-Editable text plugin made by a third-party member which can be found in the Plugins section using search.

From what I have seen, this kind of result can be acheived by using Topaz Vivacity, a Photoshop plugin that meticulously and intelligently sharpens and increases the contrast/saturation of your photos. There is no simple equivalent when using Paint.NET, but I would suggest using a combination of Local Contrast Enhancement, Sharpen+, the Hue/Saturation adjustment and Curves to achieve something similar.

Assuming you are applying a repeating pattern to your models, try using subtle gradients instead of solid colors (i.e. red to slightly darker red), and also play with gloss highlights (although this is rather difficult with a repeating pattern).

All the stars are blue, which makes it look very flat. For more realism, try using the Add Noise function to create a basic field, then use the brush tool to create a few larger ones. Also, the extremely harsh shadow does not match the rather smooth and gradual highlight. My suggestion would be to make the glow surround it much brighter (somewhat similar to what CommanderSozo has done on his planet piece).

Actually some already has. The Fill Gaps plugin seems to do just that, although judging by the example pictures, it may not yield the best results if the background is too complex.