Psion
Adventurer
At low level I doubt DR 5 will make that much of a difference - take for example a greatsword with some strength for 2d6+3 dmg,
Hold on here. "Not much of a difference"?
First off, this is an average of 10 points. 5 DR halves it right off the top. Power attack is an options (an all too obvious option, IMO, BID), but depending on the AC of the foe, that may be a limited option. Now you are HALVING the damage of this character.
Now consider: this is your best fighter here, and the best type against DR (namely, two-handed fighters). You start talking about the rogues and clerics in your group, they aren't going to be doing as much as your mainline fighter.
In my current 3e game, the non-fighters are innefectual against DR 5/+1 creatures unless they have a magic weapon.
The impact is significiant.