Having played both NWN and the BG series, I can tell you that they are not directly comparable. NWN is based of 3rd edition DnD, and BG is based off 2nd edition. A few differences that you should take note of:
-Casters, when casting a spell, do not have concentration checks. If they take any damage, the spell will be lost
-HP gain is high for the first ten levels, but then drops off greatly
-The bonuses for stat points work differently. The manual has tables detailing it, but off the top of my head, scores of 6-15 have mostly the same effect. There are differences as such:
--STR values in that range affect what equipment you can use and your encumbrance levels
--INT values in that range affect a mage's chance to learn spells from a scroll
--WIS values in that range affect bonus spell slots for divine casters
-In NWN, you could easily multiclass and change class. in BG, unless you're human, you pick a class and then stick with it forever
-There are no feats or skills (except thievery skills).
-your to-hit chances are determined by THAC0 (to hit armour class 0) and AC (armour class). The to hit roll is as such:
--roll a d20
--add the opponent's AC to the roll
--if this is greater than or equal to the attacker's THAC0, it hits
--this can lead to minor confusion when equipment offers a bonus to your THAC0 and AC.
-Non-warriors (priests, thieves, wizards) will only ever get 1 attack per round (2 if dual-wielding)
-Warriors gain additional 1/2 attacks per round at levels 7 and 14
-There is no metamagic
-Classes level up at different rates. Thieves level up fastest, and wizards the slowest.
-Saving throws are based solely off the character - there is no way to make a spell's saving throw harder
This has the following effects:
In 3rd edition, wizards were wildly overpowered, especially in PnP. In 2nd edition, High-level wizards were still overpowered, but it took them a lot longer to get there and it happened later. For most of BG1, you will find your warriors and clerics the most useful and wizards will be no use except for the occasional fight. Spells like sleep will be a great help. For the end parts of BG1 and most of BG2, you will find that wizards can do more damage than your warriors, but they are unable to sustain it for as long. In the end parts of BG2, you will find wizards are able to win fights on their own, and warriors serve little purpose other than to engage the enemy until the wizard can step in.
Also, on the pantaloon front, it's not just about finding the pantaloons, you must also keep them.