I'm not overly familiar with the website, but the few recipes from it I have come across are usually accompanied with an in-depth recount of the thought process that went into the recipe, including failed experiments and iterations, commentary on the origin of the dish and the author's experience with it, why a certain ingredient or technique was chosen in place of another, etc...

Look for Serious Eats recipes/articles about Ramen, Pizza or Yorkshire Pudding for some good examples.

It definitely looks and feels like OC (I really know nothing about Tasty, so I cannot comment on their alleged plagiarism).

Pretty regular reader from Serious Eats here. Kenji (the guy who is the culinary director), puts tons of scientific rigor into all of his recipes. What sets Serious Eats aparts from everyone else is they like to answer the 'why' of food. For example here is a super detailed story:

http://www.seriouseats.com/2009/10/the-food-lab-science-of-h...

Pretty sure you're weren't very familiar with Serious Eats when you wrote that, can you confirm this? I find it hard to believe that someone that cooks/reads Serious Eats would say that.

Asking if Kenji stole recipes is like asking if Modernist Cuisine stole recipes. You'd have to be unfamiliar with either to ask.