Lulu is a champ well known for being an incredibly helpful support in bot lane with many champions, often paired with melee champs due to her speed boost, shield and medium range knockup ultimate effect but due to this, she can take on the role of a tank during the later stages of a game where her shields and single target effects become less of a threat to the other team. After Lulu has done the job of getting her fellow champs fed in the early game, she will have gained the "global taunt" and many will want to get rid of her quickly before she can be too much of a hindrance. This is why she works as a tank ;D

Although these runes seem a bit iffy for a tank the early game ability power will go a long way to keeping her shield at a useful strength early on while also covering the early armor and magic resistance in "difficult" ranged bot lanes.

The normal start for a Lulu going for support may get the other team thinking you are going to be the full support they expect and these items give you early vision with mana and health regen as usual. After these, getting mercury treads will give you movespeed 2 but more importantly in a tank, magic resist and tenacity. Obviously if in solo queue and for some reason the other team go heavily AD based... buy ninja tabi. The Aegis is standard, needs no explanation ;D but the next items are situational. I would recommend buying at least one of the hp increasing items first, often starting sunfire as it's effect is not as useful later on but the armor and hp would be worth it anyway. I will sometimes switch out the warmogs for a second sunfire cape. Although the effects do not stack, it grants both armor and a large amount of hp which no other item but Aegis will do.

As tank Lulu I ALWAYS take Exhaust and Flash. Exhaust can often keep many of your team-mates alive in certain fights due to enemy damage reduction but it is also useful for chasing. Flash is used for chasing and tight rescue. I have to say, I have found myself flashing over a wall to ult someone in need for a pro escape or turn-around fight, it's always much appreciated when you do something heroic like that ^_^

This is Lulu, YOU DON'T NEED NO FARM!!!

Pros:
Early game support
Late game usefulness
Good for chasing and saving team members
No one expects it and surprises are always good :D
Cons:
Early game mana can be an issue even with Philo stone
Without enough team damage, the early game becomes difficult

DO IT!!! You will become a strongly dominant threat in the late game, outclassing may other tanks due to Lulu's AOE knock up on her ult and her strong silencing, no attacking effect.
Take Alistar for example, his ultimate lowers damage incoming to him, a competent team will not attack him in any way because he is the tank... makes his ult seem pretty useless in comparison to an AOE knock up and slow doesn't it? You may argue no but that is of course your opinion as this is mine ;D
Have fun with Lulu, lord knows I have!! Just give it a go, you WILL be surprised :)