5e is somewhat of a merge between 2E (i.e. BG1/2) and 3.5E, with a bit of its own innovations.
The progression level-by-level is closer to 2E with a touch of 3.5E - you're not fidgeting with skill points and other things every level like in 3.5E. But like in 3.5E, once in a while, you're given a Feat or Attribute increase (unlike 3.5E, you have to choose between the two, you're not given both). Because of this, player stats don't bloat as much in 5E as it did in 3.5E. The multiclassing system, however, is very similar to 3.5E where each level is like a lego block and you can basically configure however you wanted (as long as you meet requirements).
Weapon profiicienies, choices and statistics have been simplified (though Larian is bringing some extra diversity back in BG3). On one level, it's less diversity. On another level, you don't have clear imbalances like in 3.5E where some weapons simply have way better crit-ranges than others.
Spellcasting now has a concentration mechanic - so some spells can no longer stack, and physical damage can disrupt them (instead of needing another caster to counter them). It balances martial vs. casters a little bit, and it makes the caster's spell choices a bit more strategic. Probably my favorite innovation for 5E.
