Guys it's fifth edition D&D not 3.5E like NWN / NWN2 or 2nd like BG / IWD were. Fifth edition basic comes with four class's (Cleric, Fighter, Rogue, Wizard) and PHB core has eight (Barbarian, Bard, Druid, Monk, Paladin, Ranger, Sorcerer, Warlock). Since they mention six I'm going to assume the core four and two of the others. Personally I think it's kind of a shame but the power scaling of 5th edition isn't nearly as dramatic as 3rd edition so lets see how this works out. And yes I'm smelling future class's via DLC or expansions. It sucks but that's probably how they got the execs to buy off of funding this vs another CoD / DoA / Skyrim clone.
Personally I felt NWN / NWN2 had the absolute best engine for expandability, you could do nearly anything you wanted with it, including running a dedicated server hosting a persistent world with GM's and everything. It was a miniature MMO kit. There was a group who even made a complete Baldur's Gate module for NWN2, they working on Baldur's Gate II right now.