Well, for instance I notice they made the spells the same as BG II. Then there's the characters and their dialogue, clothing, weapons, etc etc... Let alone the dialogue and scripts... Traps... EVERYTHING.
YES they were already somewhat "there" in DA:O but they would have had to modify it to adhere to the BGII ruleset, correct?
Quoting from the GameBanshee article
"Class kits, alignments, and player voices have not yet been integrated."
The traps are just new trap effects (that, quite frankly, look worse in a lot of ways. But that is nitpicking :p) that may or may not have already been in there (thinking the fireball traps).
New spells, but I will have to see the spells in action to see if they really count as anything more than a ten-minute hackjob (if Melf's Acid Arrow, Magic Missile, and Chromatic Orb are all the same spell effect with a different color, it isn't that remarkable).
So yeah, they did accomplish quite a bit (more than most modders), but to pretend they have "everything" made already is just wishful thinking. Okay, let's pretend for a moment that they HAVE implemented every mechanic thing, creature, npc, and spell effect (which they have not, and which is NOT an easy task to begin with). That still leaves geometry and scripting, and BG2 had an obscene amount of scripting.