Since I am working on an (unreleased) converter myself I thought it might be a good idea to share ideas, and ask for ideas, about what exactly a converter should do and how it should do it.
Here are a few things that I consider essential features (feel free to differ):
[*] Ease of use for non technical users.
[*]Support all existing minecraft map formats - not difficult as the minecraft map format has only had minor changes over the years.
[*]Fast conversion - there are limits to how fast we can convert since minecraft region files need to be decompressed, and minetest mapblocks have their node data and meta data compressed, however conversion times can be reduced if we avoid lots of needless calculations. For example if the minecraft block x,y,z in a minecraft section should be translated to a certain position in a minetest mablock, well the block x,y,z in all sections will have the same translation, calculate once then in future just look up the answer.
[*]Support minetest game and mineclone 2 - I consider this the absolute minimum level of support, funny thing is mineclone 2 is easier to support than minetest game but tends to get ignored for some reason.
Things that would be nice to have:
[*] First time you load the converted map you should spawn at a sane location, this is generally quite simple as the spawn position is stored in minecraft's level.dat file, the values in level.dat can be overridden but that's quite rare in my experience and can be detected anyway.
[*] Fix the water level. Sea level is at 62 in minecraft which is a tad awkward as it's not a multiple of 16 (size of sections and mapblocks). Fixing to within 2 nodes is trivial, fixing sea level exactly is a little more involved but doable with only a small impact on conversion times - is it something that's worth doing?
[*] Chest contents. Should the contents of chests be converted, again not difficult to implement and has only a small impact on conversion times but is it really needed?
Anyway, any features you think a converter should have etc let me know here.