We have all these different building materials but they're all the exact same thing. The only difference as far as the player is concerned is the look of them. This needs to change. Aesthetics are all well and good but it's called Survival for a reason and part of surviving is maintaining defenses and shelter against hostile elements.
Zombies are ravenous. Zombies are single-minded. Zombies want to kill you and
1) Zombies will spawn in groups as opposed to alone. Hordes would consist of four zombies and would function the same as a current zombie, merely together.
2) Groups that come within five blocks of one another will join into a single larger group (horde). Hordes would be limited to eight zombies (or two groups)
3) Zombie groups will possess the ability to break down player-placed blocks (planks, cobble, dirt, wool, etcetera) at twice the speed of the player's fist (I.E., a player takes 10 seconds to punch out cobble, a zombie group would take 5). This speed will double for a horde (so it would take only 2.5 seconds).
This would inject more survival into Survival mode, in addition to giving building a purpose beyond looking pretty, as well as a purpose to traps and house defenses which are little more than aesthetic dressing in single-player at the moment.
*To appease the building-oriented players who would rather focus on the looks of things alone, limit this to higher difficulties or make it a toggle option (Zombie Hordes: On/Off) prior to world generation.
And yes, I did just call you a giraffe. You know who you are. Quit eating my trees.