(I apologize in advance for a downer of a comment, especially for the parts that are poorly argued.)
In general, I think the risk of important but unknown nutrient deficiencies from cutting out a whole class of foods is probably much more important than the animal welfare benefits if you are spending much of your life working on good projects. This is a concern with vegetarianism, but it seems much more severe for veganism.
I haven't looked into the literature in creatine in any depth (largely because my diet is long on creatine), but the evidence for material cognitive effects looks weighty enough that I would want to be careful before considering returning to a low-creatine vegetarian diet. It seems like this should probably get more weight, unless everyone else knows something I don't.
Based on rough estimates it seems like being a vegan is not going to be worth it on consequentialist grounds, at least for those who would e.g. be willing to pay an additional factor of 2 premium to eat meat periodically. For such people the question is: if raising an animal generates $X of economic value, can you generate a welfare offset for a small multiple of $X? It would be quite surprising to me if this weren't possible (e.g. just subsidizing the creation of a small number of more humanely raised substitutes should work, which I would guess is orders of magnitude less effective than realistic approaches). I haven't thought about this at too much length, largely because I think that the effects on animal welfare today are not too important either on contractarian grounds nor on utilitarian grounds (since they seem to have a minimal long-term impact, and the direct impacts are small compared to long-term considerations). But I do think that given the basic economic logic the burden of argument rests with the vegetarian advocate.
If it's a bad idea on consequentialist grounds, I'm not convinced it's a useful signaling exercise. Alternatives like "cut out the worst offenders," "reduce consumption by 90%," and "buy humanely produced products" seem much better both on consequentialist grounds and (consequently) also signaling grounds.
I second the recommendations for powersmoothies elsewhere (though I drink mine with milk, which I find more delicious and seems probably better health-wise).
Several comments raised the concern of unknown health issues with a veg/n diet. While I think unknowns are exceptionally important in nutrition given the low-quality evidence that exists overall in the field, I don't think that's a strong argument against veg/nism because so many veg/ns have already lived and been as healthy, if not healthier, than non veg/ns. And we can say the unknowns cut both ways here, both with unknown benefits and unknown drawbacks to veg/nism.
In general, I think we should have added skepticism for arguments against veg*nism due to the high self-serving bias we have to continue our current diets.
Also, http://www.huffingtonpost.com/2013/06/04/vegetarians-death-premature-longevity-live-longer_n_3380781.html
And we should have added skepticism for health-based arguments for vegetarianism that are made by people who are vegetarians for non-health reasons, as it would be extremely convenient if making a massive dietary change for non-health reasons turned out to have no major health issues.
I don't see it as a massive dietary change. If anything, it's closer to our historical diet (which, AFAIK, generally consisted of lots of plants with little animal products). Also, I don't expect most dietary changes of a similar scale to going veg/n (say, cutting out grains or cutting out fruits) to have major health issues. So it's not "extremely convenient" to me.
Also, it might even be extremely convenient if making a massive dietary change for non-health reasons turned out to have no major health benefits!