I was just looking through an old, unfinished Let's Read I did of the D&D Next playtest, and I would say the following were throughlines for the whole of the playtest:
-d20+modifiers vs. DC, with Advantage/Disadvantage
-Bounded accuracy
-"Chunking" of character generation, and in a wider sense, of complexity
To explain the last, while the implementation was different at various stages of the playtest, there was always an attempt to contain complexity by divying up lower-order choices among higher order choices. For example, instead of making players choose all their skills and equipment a la carte, or putting all a character's skill choices in the class basket, acquisition of skills and equipment was divided between Background and Class. Likewise, class complexity was split between the class and the subclasses, as well as by level. It seems to me that every clone of 5e likewise follows this design philosophy. Implementation of specific components may differ, but the basic chassis of Background/Class/Subclass is always there.
In the greater sense, complexity in 5e is always "opt-in." Choose whether you want to use feats. Choose whether you want to use a battle-grid. Choose whether you want to include tactical options such as marking and flanking. Choose whether you want to play a simple, straightforward character, or a more complex, optimized character; there may be differences between the two, but they are within acceptable tolerances. And this is again reflected in 5e-derived games, where you can strip it down to the minimum to get Shadowdark, or turn it up to get Level Up.
There is one last thing that was NOT present through most of the playtest, but ended up being the glue that held 5e together.
-Proficiency bonus. It seems so obvious now, but throughout D&D's history, and indeed throughout the playtest, there have always been varied bonuses of varied scales for different characters, for different aspects of the game (e.g., combat vs. skills vs. saves). Combined with bounded accuracy, this +2 to +6 over 20 levels provided an elegant unification of bonuses for all characters and aspects of the game, while still providing for distinct differences in abilities/skills.