Windows: In your users documents directory. (e.g. "C:\Users\sithlord1337\Documents\jk2mv")
MacOSX: In your users "Application Support" data e.g. "/Users/sithlord1337/Library/Application Support/jk2mv"
Linux: In your users home directory. (e.g. "/home/sithlord1337/.jk2mv")
Portable Versions: In the current working directory (usually where you installed jk2mv).
Windows: Left/Right Windows key
MacOSX: Command key
Linux: Depending on your keyboard the Windows key or Command key. If it fails you can try Alt + Tab.
Yes, the new fonts introduced in JK2MV 1.2 can be disabled.
For the console font: you can either choose "Original" in the menu (Setup > JK2MV > Console Font) or manually set "r_consoleFont" to 0.
For the menu font: you can set "r_fontSharpness" to 0.
In the past (during the time of jk2mf) we noticed that a couple of servers provided downloads with content that "replaces" base-content, for example modified versions of ffa_bespin. Providing these "replacements" wouldn't have been much of an issue if all of those servers were running their servers using "unique" folders (like: "Twimod_dark_server1" instead of just "Twimod") and provided the downloads for those folders only. Most of these servers however placed their "content" in the base-folder, which made the clients download the file into the base- folder and load it on every server they connect to. We have seen servers "replacing" textures, maps and animations, leaving desperate clients, who had no way to find out which file caused the change and re-installed their game. To solve this jk2mv adds the prefix "dl_" to downloaded files and only loads the files starting with "dl_" when a server is "referencing" the file (referenced files are those the server currently uses itself, like the pk3 of the current map). This way content like modified ffa_bespin maps that have been downloaded into the base-folder will only be loaded on servers that use the same pk3, hence reference it. This also solves the issue of different versions of maps (for example expedition) preventing players from joining servers with a different version (as long as the players autodownload the version the server offers). If you want to enable a specific pk3 (like christmas_bespin, because of the santa skin) on every server simply remove the prefix "dl_" infront of the name or - as of jk2mv 1.2 - go to "Setup > Downloads", select the file you want to make permanent and click the "Make permanent" button (or if you want to delete the file click on "Delete").
tl;dr: To avoid conflicting mods / maps etc.
All qvms (cgame.qvm, jk2mpgame.qvm and ui.qvm) are ignored if found in pk3 files in the base directory. This is due to the incompatibility of these files between 1.02 and 1.03/1.04 versions. You can prefix the name of your pk3 with "o102_", "o103_" and "o104_" to specifiy which version your pk3 is targeting. So name your file e.g. o102_mymod.pk3 for 1.02 code mods. This is only required for code-mods. Files with the prefix "dl_" are considered downloaded pk3's and are only loaded if the server requests them to be loaded. You can manually remove the prefix to load them everywhere (more information in the question regarding why downloaded files have the "dl_" prefix).
As of JK2MV 1.2 the pk3 loading rules have been updated, information about additional rules can be found here (the old prefixes still work).