The only way to play morde now is a rate-based hero, and you need AP for that. You can't just go in and slam down a E/W and have so much shield that you are invincible. Instead, you need that shield to be constently generating. To do that you need full CDR and high AP with good amounts of armor/MR to compliment.
CDR boots are great on him
Hourglass is great (Armor/AP)
Abyssal Scepter is great (MR/AP)
Deathcap is great (30% AP, more shield "rate")
Banshee's Veil (MR/spell negate/AP)
Guardian Angel if you are being focussed first.
His problem is that CC just absolutely demolishifies him. You NEED your shield to be constantly generating, and if you are CC'd you are not generating shield.
I think Rylai's is horrible on him now, because his job isn't to chase/snare dominate. His job is to initiate, generate massive shield, die in a glorious fashion while your real DPSers finish the fight. The more AP you generate per unit time, the more dmg you can soak up before you die.
Why you shouldn't pick him? Because he kinda sucks early to be honest. It's a huge hamper to your team and by picking him, you are asking your team to carry you a bit at first.
Why you should pick him? Because his rank 3 ultimate on a fully farmed carry can still single handedly win the game. Get your team to focus the carry first, ressurect him and smash with him 6v4.
His R is still OP at level 18. Getting there is the problem.
Btw, he can still farm/push like a pro, so midgame, you can try stall-out tactics like have 4v5 defensive base-turret defends while you ward up the map and push top/bot while your team defends 4v5. Then once you are all 18 and fully farmed, you all push, you R their carry, and win. That's how every game should go. Getting your team to 4v5 defend under your turret is the hard part though.