Apart from eschewing animal flesh (including fish!), I avoid eggs, dairy, leather, silk, fur, HFCS (high fructose corn syrup)...but not so particular with honey and wool, and sometimes fish sauce, pearls and gelatin (living with omni gourmand parents), though I'm borderline with the latter three, so I'm not vegan...yet.
Apart from dairy cheese promoting cattle murder, et al., I am lactose-intolerant, so I get stomach pains and migraines and diarrhea and gas, to name a few effects.
Being from the Philippines, I thought I'd have to make do with no cheese, and so far, it's been OK.
Then Healthy Options, our local mega-healthfood store brought in Nutritional Yeast, and I was one of those who placed in an order. I was given a lax recipe of soymilk+mustard+margarine+nutritional yeast by a fellow community member over at Livejournal. Holy cow (pardon the phrase), it was orgasmic. I next tried it with fried tofu and Heinz Tabasco Ketchup, and it tasted like eggs without the murder. Mom looked at my "cheese sauce" with interest, and when she found out what it was, and that it was nutritious (um, "nutritional yeast" must be called that for something?)--especially high in B12, she was all for it. I mean, the recipes one can use with it are endless! A next one to try would be steamed broccoli and nutritional yeast. As she said, "So we're not limited to tofu anymore!" Um, I never was, Mom, but coming from an omni...fine. xP
Thus my first foray (and hopefully not my last) into Nutritional Yeast Heaven.