It's impossible to live and thrive without being detrimental to other organisms - for me, the idea is to accept the inevitable cycle of struggle and violence, while minimizing the _exploitative treatment_ of other beings [plant or animal -- easier said than done with regard to plants -- one easy step is to include as much fruit and perennial leaf matter in the diet as practical [i.e., the plant is treated more like something to be nurtured rather than merely 'grown and discarded']
There's still the level of consciousness and intelligence to look at.
I eat everything but octopus/calamari. I think they are too intelligent to eat. I've seen smart cows, and if I didn't grow up eating beef and it wasn't such a great source a protein, I would cut it out too and just eat chicken. For me it's where do I draw the line of intelligence of what I want to eat. The less likely the animal can comprehend what's going on, the easier I feel about it.
I can't eat Octopus, Pigs, Dogs, or any type of Ape because they are too aware of the process.
Oddly I don't have the same aversion to Dolphins or Whales. They may have a comparable amount of intelligence to animals that I wouldn't eat, and a dolphin is even cuter than an octopus, but for some reason I don't have a problem identifying them as food. Sort of like Sheep and Goats, they are cute, but also clearly labeled as food in my head.