Physicality and Late Game Defensive Prowess
On defense, Love and Pek's inability to disrupt or switch on pick-and-rolls hurts, but more subtly is that fact that between Rubio (or JJ Barea when Rick Adelman decides to go against medical advice and play him over Rubio), Martin and Brewer's lack of strength also prevents them from taking advantage of the same leniency which victimizes them on the offensive end.
As an example of the kind of play the Wolves simply can't make defensively in the late game, Saturday's Raptors/Blazers tilt had a perfect example. Despite an extremely impressive comeback in the second half of the second game of a tough back-to-back, the Raptors were unable to get a look in the final seconds because Wes Matthews was able to use his strength to force a turnover on the final possession:
If Brewer is physically stronger, he bodies Green at the wing, and prevents him from even cutting into the open space for the shot.
As an aside, the beginning of the above play demonstrates the problem of Love's pick-and-roll defense in late game scenarios as Goran Dragic is easily able to turn the corner on Love's hedge: