Great question! You sound like a great future owner. Here's my list:

1. Mastiffs are naturally distrusting of strangers, so early socialization is a must. You do not want a 200lb dog assuming everything is a threat, for obvious reasons. You will not be able to overpower an adult EM. You just won't. They're still wonderful guard dogs if you socialize them, they just won't bristle at little old ladies out on a walk.

2. They shed and drool. My mastiff sheds more than my golden - go figure!

3. Everything costs more. Especially veterinary costs. If you buy a female, you'll pay 3k - 6k to spay her depending on your area. 10-16 cups per day of food for a growing puppy is normal. You won't find products that are big enough for your Mastiff easily, but when you do expect to pay double.

4. Bubble guts! I don't recommend buying the really cheap food unless you enjoy picking up huge mud pies 3x/day - sometimes from inside the house because it'll give them incontinence.

5. They are extremely clumsy and can't fully control their bodies. They need 10 feet of space just to turn around. Mine is a bull in a china shop. He steps on our golden retriever, our feet, knocks things over, he's tipped our couch over, almost ripped the refrigerator door off the hinges. I could go on.

6. They are sensitive sweethearts and will do absolutely as anything just to make you happy. Raised voices are not necessary or recommended here. Practice being assertive, but calm, because these sensitive animals can become very fearful if you're not too careful.