Mord is hard to play correctly. He pushes minions so easily. But he is not a burst dps character. He is a giant AE dot really.
Get a frozen mallet so that once you get ahold of something it can't really get away. Get a bloodthirster, the lifesteal + your sheild working off spell lifesteal makes you VERY durable. Sunfire cape + Chilling Death sheild makes you amumu's big brother, plus your Mace of Spades AE with Siphon AE. Basically in a team fight you just dish out 200 Damage/Second to EVERYONE. Not many champs can sustain that level of damage over a 6-10 second period, which is long enough for a team fight to go down and leave you on top.
The only thing that shuts Mord down is the same thing that shuts everyone else down:
Stun/Fear/Silence/disables in general. Other than that, if you are not owning with Mord, you are doing it wrong.
OH and owning = your team winning, not you having the best K/D ration. I typically go 1:1 K/D with about 12-24 assists.