What sets Nemesis apart for me is the fact that it's a complete assault on the senses, and it begins as soon as you enter Forbidden Valley.
The sound it makes is by far the most memorable I can think of, and I put that down entirely to the way it echoes off the surrounding pit. It amplifies the sound, and changes it slightly to make it sounds like a roaring creature. Combine that with the sound of the waterfalls just magnifies it further. It's forboding, yet draws you towards it, all at the same time.
Then you've got the appearance. The theming over the station is awesome. The red dye in the waterfalls are a simple but effective way to theme the whole ride area and draw it all together. I think the way that you can't really follow the path of the track very easily also helps. The way it weaves in and out of the rocks, and under itself make it hard to guess what it's going to do before you get on. I also love the fact that it has that kind of ampitheatre thing going on with the steps that go from the station where the single rider entrance is) up towards the main entrance. It focusses your attention on the beast.
I also love the way that you can feel the ride as it goes past. Most notable is if you're queuing on the bridge towards the station right infront of the vertical loop. As the train swoops through the loop and down under your feet, the gust of air that rushes past is pretty intense.
All of these elements come together to make it one of the most imposing coasters that I can think of. This is why I love it so much. By the time I actually get on the ride, it's whipped me into a frenzy of excitement and expectation like no other. Of course, all of this would be pointless if it didn't have the ride experience to back it up, which fortunately it has. The intensity and relentlessness not only make it fun from start to finish, but also perfectly match the expectations built up during the queue.
Sure, I've been on other inverts (and other coaster types) that are imposing, or intense, or great fun, but none of them put butterflies in my stomach or a smile on my face quite like Nemesis can.