The addition of a legitimate WR to the offense will only help the running game. Crabtree has the potential to change the way defenses play us. He will require a safety help at all times, which will limit the number of players the defense can have in the box.
Couple Crabtree's talent with V. Davis' athleticism, we have the makings for a potent passing attach. Both Crabtree and VD will pose mismatch problems (assuming that the new OC can get the ball into VD's hands) that the defense has to account for. Our offense should not see any 8-man fronts anymore.
I see the 49ers reverting back to a style similar to the Walsh days. We will use the pass to score points in the first half (using play-action instead of the WCO) and we will use our power running attack to wear down the defense in the second.
On a side note, Crabtree will also help our red-zone offense. He is a big strong WR that can beat a DB one-on-one at the goal line. For instance, look back at how the NY Giants offense stalled last season when Plexico went out. A big WR is a must for redzone offense. Look for us to score more TD's this year.