I think the criteria should be that the amount you have to lose is overwhelming to you. I don't think the number matters so much. For some, yeah, 50lbs is totally overwhelming. Others maybe not so much. I feel that what bonds us together is that "Oh my goodness, where do I even start?" feeling.
I also think the timeline is important to consider. Most of us are looking at 1year+, minimum, to reach goal weight. We are all in it for the long haul.
But I don't think this thread needs hard and fast rules or anything. If you feel you have "a lot" to lose then you belong here.
I think the risk of very low calorie diets is that it is so hard to maintain. As we walk this journey we need to take heed of those who have walked it before us - and they will all tell you that taking the weight off is the EASY part. It is the maintaining that is hard. So although This Diet, or That Diet will probably work in the short term, it is only through sustainable lifestyle changes that we will be able to succeed for the long term. Some may use a special diet plan to kick start things and then move onto a more sustainable healthy eating plan going forward, and if that works for you, great. Others just start with the healthy eating and are content with a slower loss. I am in the second camp for now.
Realistically, the first thing I need to do is stop gaining weight. Even just maintaining where I am at for a while will be a victory.