I enjoy reading letsrun, but I usually just lurk, because I am not, and I never will really consider myself a runner. Over a decade ago, I finished in 28+ hours at Leadville, and I believe that I will finish sub-3 at Boston this month. I don't think people on letsrun actually hate ultra runners, or even dislike ultra runners any more than slow joggers. Every year over 23,000 high school kids run under 5 minutes in the mile, and I would wager a very high percentage, say >95% could run under 9 hours in a relatively easy 50 mile run. I say that in part, because I never ran under 5 minutes in the mile in high school, and right after high school I went and did a 50 miler on minimal training in a bit over 9 hours. I had a lot of fun, but I knew then and still know now, it was no big accomplishment, and likely less impressive than a 4:59 mile run, which would, understandably, get zero respect on these boards, because, it is damn slow compared to everyone else who actually runs.
Yes, comparisons are difficult, but any reasonably fit runner should be able to go under 6 hours in an easier 50 mile trail or road race. Jim Walmsley, for example, wasn't an especially fast college runner, sub-14 minute 5k, so not especially fast, but definitely very legit, and he ran a 5:21 at the JFK 50 miler. I haven't looked over the threads recently, but I believe that Walmsley has gotten some respect on letsrun for his performances. Yet, compared to someone like Galen Rupp, sub-13 5K, or Eliud Kipchoge, also sub-13 5K, he really is not a very fast runner, and I doubt anyone would believe for a second that Walmsley would not be totally destroyed by Rupp or Kipchoge in a 50 mile race.
So, in short, I don't think there is any hate for ultra runners on letsrun, I just think there is justified criticism of many ultra runners who think they are special- or actually, even runners, when many of them are really just out hiking. I hope they had a good time out on the trails, I know that I always do on ultra hikes or when playing and jogging in the woods, but I don't try to delude myself into thinking I am a runner or that I have done something worthy of attention or acclaim, in comparison to the many, many people on letsrun who have actually run under 4 minutes in the mile. The funny thing is, I believe that most "ultra runners," would actually be a lot faster (relatively) if they focused a lot more on running fast for short distance. Anyways, that is my hope going into Boston!