IWD2/IWD/PoE:BB---------------------------------BG---------BG2-------------------------PS:T
IWD2/IWD/PoE:BB: Non-existent PC/companion story and relationships.
BG/BG2: PC/companion relationships and stories throughout.
PS:T: Heavy PC/companion relationships and story.
As far as I can tell at this point in the beta, PoE is most closely resembling IWD2 and IWD. Most people seem to want it to be more like BG/BG2.
The major thing lacking in PoE:BB that would push it towards the BG games is a main character and companions with an actual story behind them. Even if they aren't in the main game, but if that is in the main game it should be an aspect that is at least represented.
The IWD games were about the story of the world around the party not the party itself.
Though, PS:T is in a league of its own, it and the BG games were about the journey of an individual character and their companions.
The feel of the game is completely different when playing with a generated party vs. a party of characters; a party where you care about their past, their future, and are emotionally invested in them.
I know the beta is the beta and is likely completely devoid of story spoilers, but it's scary playing the beta when things you really wanted in the game are not even remotely there.