Could share some tips besides what's in those (very very cool) videos -- I may not have as much experience as someone like DarkFlameWolf or Sirius but if you've seen my screenshots in Xfing's thread you can see I've put some practice in, anyway:
-Pressing "Home" centers your view on the cube you've got selected; it can make all that twisting and panning much less intimidating especially when you want to work on two far-apart sections of something *enormous*. And if you're working on singleplayer missions that will probably be exactly what you're doing.
-Saving your missions into a separate folder from your game folder and copying them over when you want to test is a little more time consuming but it makes organization and backups a lot easier, something you'll be glad of when you've sunk twelve hours into a project and you get a freak editor crash.
-Find the keyboard shortcuts for those options in the dropdown menus (usually they're in the dropdown menu next to the item). It may only save a second or two each time but over the lifetime of an editor file those seconds add up. Memorizing them should be the first thing you do. Once you've got the instinct you can focus on what you want instead of how you're doing it.
-Point mode can be a chore to work with but if you're a careful planner you can often do exactly what you want to do with just side, line and occasionally block mode.
-I tend to set the "mine" movement ratio in settings to 2, this makes most of the big editing operations on cubes twice as fast. I only really need to turn it down to 1 when I have a very very fine adjustment to do, which isn't really that often.
-There's no grid here, so the best way to make your cube adjustments line up is simply counting keypresses. If you're as distractable as I am I find it's very useful to have a piece of music on in the background so you can count to the beat, and not to leave an adjustment half-way done for any period of time when you could lose track of your count. I also find the "other cube" (spacebar) can be a very good visual aide when you're making adjustments to one side that can affect others. This may seem silly but it's absolutely vital when you need to have two completely separate sections of your mine line up for an interconnection.
A few tips I have for singleplayer specifically:
-The first thing I try to think when I'm blocking out an area is, in very broad terms, "what do I want to happen here?" It's tempting to only think about what will look best when you're building the geometry, but if you want a big puzzle or action piece to happen it's always, always best to have the geometry adapted to it from the start instead of having to adapt your set-piece (for lack of a better word) to the geometry later on. And it's generally possible to flesh that functional piece of geometry into something that looks good too. This is a little less important for multiplayer levels though it should still be in your mind. It's extremely important for boss arenas because the way you make a boss battle engaging when you don't have a scripting engine to work with, is to direct the players' attention and actions in an interesting way, and then rewarding them for going there or doing that.
-Don't use too many lights unless you don't want your map to have any atmosphere at all. When you're starting out, lighting and texturing as you go is a good idea so you can get that feedback on whether you've got what you were looking to get or if you still need to make some tweaks. It's much easier to get all your geometry to line up properly if you tweak as you go instead of doing it all at the end.
-When you're using the align-texture buttons that affect adjacent sides that share a texture, if you've got only the "use first" tick-box checked it can cause faces where you've got secondary textures like lights or warning stripes or signs or even doors to become very screwed up. To avoid manually resetting everything that isn't supposed to be a flat wall you can check both the "use first" and "use second" boxes before you do your alignment and it will leave those faces with a secondary texture (or a different secondary texture if you really like your secondary textures) alone. You could also start your alignment from one of those faces but if you've got more than one face in the room with a secondary texture on it the rest will still get screwed up.
-Nothing turns out exactly the way it looks in your head. Use that as an opportunity to make it better than what you imagined, instead of settling for worse.
-More of a personal note for me; if you are trying to create atmosphere keep in mind that the introductory music tracks for both D1 and D2 are not particularly scary and can sometimes ruin a frightening atmosphere for those who like the music on. Don't be afraid to take one of the game's other hmp files, rename it to game01 and throw it in your HOG. There are quite a few to choose from and it gives you music for a single level without having to work with the midi format and try to figure out how to convert something else to an hmp that doesn't crash the game. Ogg is an option, but it doesn't work with the base game, only the source-ports. Each of those has multiple different and somewhat obscure methods for adding them into an actual mission. Unless the music's a big part of your vision it's best to pick whichever of the base tracks you think fits best and go with it.