As a ruleset for a megadungeon setting, it does play very differently than AD&D 1e and Blood & Treasure, our two previous rulesets for Montporte. Here are two quick observations after one night of play:
1. D&D 5e dungeon play involves less resource management that AD&D 1e. With the rejuvenating abilities of short and long rests, plus a highly modified Vancian magic system that presents more flexibility for players, players have to worry less about running out of hit points or spells (for a similar observation involving D&D versus GURPS, see Patrick Halter's (Renovating the Temple) blog post, Maintaining Encounter Balance). With the earlier editions of D&D, resource management (in the form of hit points and spells), is usually a critical component of megadungeon play.
2. Turn by turn combat tactics matter more in D&D 5e than in AD&D 1e. In AD&D 1e, combat is primarily an exercise in attrition. Tactics matter and can, on occasion, determine combat encounter outcomes but usually the raw abilities to dish out and avoid/absorb hit point damage matters most. With D&D 5e, players have a wider (but still limited) number of meaningful tactical options at their fingertips. And creatures have have more options as well, which can create havoc for careless players. At the same time, magic spells often require concentration and/or a "to hit" roll. This increases the unpredictability in combat, which players have to account for in their tactics.
Despite these differences, D&D 5e is still very much in the D&D family. For example, the speed of play in a dungeon setting is closer to AD&D 1e than 3e/Pathfinder and 4e. We were not bogged down by too many options or modifiers. We had surprisingly little need to stop play to look things up in a book. Most importantly, we enjoyed the session.