Numerous questions and answers refer to the design goals of 5th edition, and if a reference is given, it is to a no-longer valid link at wizards.com. Are the design goals of 5e currently available, and where?
I could not specifically find an article about plain language, but filtered for Legends & Lore, this captures Mike Mearls' and Monte Cook’s weekly(ish) blog posts about the goals and design of DnD Next (5e) from the announcement in January 2012 up through 4/2013. The Player’s Handbook was released 8/2014, so there is some missing time, but many of the interesting original articles about the design goals are there.
For example, this article titled RPG Design Philosophy makes points like these excerpts:
The rules shall make things easier for the DM. This might seem like a funny concept, but it’s something that is a huge distinguishing point between RPGs and other types of games. In an RPG, the rules should help move things along, serve as a useful tool for the DM, make things clear and easy to understand for players, and enable the sort of creative, unpredictable, and evocative gaming that has kept tabletop RPGs going for 38 years.
The rules shall balance character options, within reason. D&D gives players classes, races, spells, and other options to build characters. Although the game isn’t competitive, it’s supposed to give players a chance to take on a role as an adventurer in a fantasy setting. Some people want to be wizards, and others want to be warriors. The game should ensure that no single option, or small group of them, completely overshadows the rest. The game is about the adventures of fighters, rogues, wizards, and clerics, not a wizard and his or her lackeys.
This article D&D Next Design Considerations touches on some of their high level design goals, and here is a portion:
Goal #1: Reunification through Common Understanding
As part of the design process, the R&D team must boil down the RPG into its most basic component parts. Using those rules elements, the team must then build an easy to understand game system that incorporates the most iconic elements of D&D in prominent roles. Anyone who has ever played any version of D&D must recognize and understand its most important elements.
Goal #2: Reunification through Diversity
Traditionally, D&D editions have focused on specific play styles. This approach has fragmented the community over time. The next iteration must stretch the system to cover a wider variety of play styles through character and DM options. By looking at past editions and incorporating their elements as core or optional rules, we can allow players and groups to place the focus where they want it.
Goal #3: Reunification through Accessibility
D&D has traditionally required large amounts of time, a large play group, and a sustained commitment. The design process must focus on play time, group size, speed of play, and length of campaigns, with an eye toward reducing the minimum required from each area. Players who want a longer play time and so forth can easily scale up the game to meet their needs and opt into the various rules modules we'll provide or that they'll build themselves. However, our standard goal is to remove minimum group sizes, allow for a complete adventure in one hour of play, and satisfying campaigns in 50 hours of play.
There are too many articles and too much information to quote each one. You can filter and navigate the whole archived list of 3307 articles with working links from what I’ve seen.
Not sure why, sometimes the links don't work on the first try, but they do work eventually for me. Getting little off topic, but this is too much fun to dig into. This archive goes as back as an article that is dated 2002 and has a working link to a 2008 3.5 FAQ 116-page PDF with absolutely charming questions, like:
What kind of magic item slots does a dire wolf or rhinoceros have?
Spoilers: Necklaces, amulets, barding, and bracelets.