This is my chance to whine, or for you to vicariously enjoy someone else's misadventures and suffering. Hikes can qualify as Hikes From Hell for a number of reasons. And because it was a Hike From Hell does not mean it wasn't a good hike, just that it had some serious challenges as well. A hike can qualify based on length, bugs, weather, or other natural or man-made calamity. So without further ado, but not in any particular order:
Hall's Creek Narrows: This is approximately a 22-23 mile round trip. I backpacked the first mile or two down into the canyon to set up base camp and reduce the length of the hike the following day. I didn't know if I would find any water on the trip, so I carried enough for the evening I went in, the next long day in desert heat, and the third day coming back out. The back was so heavy (because of water) I could not lift it. I had to moose it onto my back on the ground, then stand up. The trip in was in August, 100+ degree F heat in the direct sun, going down a steep trail - brutal. The next day I did the 18-20 mile round trip to the narrows and back to base camp. First part going out was easy, made it to the narrows while it was still cool, spent a good chunk of the day in the cool narrows (there was water) and then in late afternoon headed back. But now the temperatures were again over 100, and the deer flies were out and hungry, so I couldn't stop for about 6-7 miles in the scorching heat without getting eaten alive. The narrows were great, glad I did the hike, but it was tough.
Long Range Traverse, Newfoundland: 6 day map and compass hike in "arctic alpine conditions". It rained everyday, especially just as we were starting to set up camp, the entire route was boggy, the black flies were constant (I ended up with one big long red welt along my hat line). They have dense low shrubbery called Tuckamore (sort of like Krumholtz for those who know what that is, except a bit taller) that forms almost impenetrable hedges. This often resulted in extensive detours or sometimes just having to wade through it. Since it limited visibility to inches in a few places, combined with the general bogginess of the land, this resulted in a few surprising steps: every now and then you would step into a 2 foot deep pothole and drop out of sight of your companion! Certainly a feeling of accomplishment to navigate through this low relief region with no trail for 6 days, but other than that, not worth it to me.
Grand Canyon: backpacked to the bottom, spent the night, then back out. To me, the spectacular part of the canyon is the view from the top. As you go down into the canyon the views get less and less, and become less impressive (other canyons are different, being in the canyon is the cool part, but somehow for me the Grand Canyon was different). So having gotten myself to the bottom, without what I considered a rewarding view, I looked up and realized I have a vertical mile (5000') to drag my carcass back out. Need for the macho of "I did it", but otherwise, just a lot of hot, hard work - not worth it.
18 miles in a day with full pack: This was part of a really neat backpacking trip to Mt. Assiniboine in the Canadian rockies. Overall the trip was great, the views terrific, etc. Yes we did have to wait through deep snow over some passes, got rained/lightening'd on, then snowed on in the night, and attacked by killer porcupines intent on eating our packs for the salt from our sweat, but it was well worth it. However, on the last day we did 18 miles, instead of doing 2, 9 miles days. We got to our planned camping area by about 2 in the afternoon (having already crossed 2 mountain passes in the morning), to find it pretty well owned by bugs (mosquitos, black flies, etc.). This was early June, the days are LONG, so we pressed on and eventually decided, "Why stop?". Definitely a worthwhile trip, but 18 miles with a back in one day is tough.
Lower Fish Creek: This was a short (about 6 miles) hike in very hot (100 degree F) weather. That by itself was not bad. What was bad was no trail, heavy vegetation, and ferocious deer flies. There was a stream in the canyon bottom, sometimes up to knee deep, in a sunken embankment, and heavy stands of vegetation. I was looking for Anasazi ruins, which were only visible on the cliff walls from the canyon bottom. Getting up on top and walking on the slickrock was wonderful: easy travel, no insects. But to see the ruins I had to stay in the canyon with difficult travel and deer flies. On top of that I had a bad blister on my little toe from a 19 mile hike the day before. I was wearing long pants, a long sleeve shirt, my hat, and had my bandana tied around my ears, neck, and forehead to reduce exposure to the deer flies. Even so, I had to walk with both hands waving back and forth in front of my face at about the speed of windshield wipers. If I stopped for a moment the flies would settle on my hands and face and bite. I was just waving my hands randomly, back and forth, and would hit 2-3 flies on every pass.
Any 20+ mile day hike: I've done a lot of these, and for the most part they are enjoyable, but the length can wear you down. A partial list includes The Back Tusk in British Columbia (absolutely spectacular!), Buckskin Gulch (awesome, but deep and narrow enough the water is dang cold!), Betatkin (actually a nice pleasant hike except the stream you are continually crossing is 90% cow piss), Berg Lake at the base of Mt. Robson, Canadian Rockies (especially coupled with a low cloud cover resulting in no views at all).
Looking back at this list, I think the primary thing that contributes to a real Hike From Hell is bugs. The long hikes by themselves are usually tolerable, the desert heat can be tough, rain is a drag, but they all pale in comparison to assault by these little critters trying to suck you dry.