I don't know why people are afraid of Nirnroot.
It's only a horrible quest if you actually go around and collect the bloody plants, but that quest was imagined to be a quest that you complete little by little on the way (at least that's my theory).
I'd say it's kind of a boring quest that's really kind of unrewarding seeing as all you get for your troubles is progressively stronger versions of a potion that wasn't all that useful to begin with, but that's rather irrelevent, the issue was not the plant itself, but the fact thatthere was a quest where the entire goal was to collect it, many samples of it. and let's face it, collecting plants doesn't usually make for a very exiting quest, and the Nirnroot quest requires you to do a lot of collecting plants, and a fairly rare plant at that, it's understandable that the quest annoys people, but Nirnroot being in Skyrim doesn't mean we'll have an identical quest, I doubt Bethesda would reuse the exact same quest even to the point where your collecting the same plant, at the very least, they'd need to find a different plant for you to hunt for.
I don't think they were Nirnroot though, despite loads of folks here claiming they saw it - Nirnroot is supposed to grow in wet areas and this is consistent with the environments where it was found in Cyrodil in Oblivion, whereas the areas in the Skyrim trailer where you could see harvestable plants looked much more arid with less waterlogged soil. So I hope they aren't Nirnroot!
The plant people are saying is a Nirnroot IS growing next to water, it's in the part of the trailer where the player stabs someone to death in first person view by the shore of what looks like a river or lake, in any case, its definitely a body of water, the water in it may may look bad, but not so bad that I can't tell it's water.
I can't guarentee that it's Nirnroot, but it certainly looks similar and is growing near water, so I'd say it's likely.
Aside from what looks like a Nirnroot, I can't really identify any particular plants seen in the trailer myself, I couldn't make out what every plant seen in it looked like (Not that I was paying that much attention to individual plants to begin with.). Besides, it wouldn't necessarily be easy to tell just by looking at a plant if it can be harvested or not, and Skyrim is likely to have many plants that were not in past games, so even if we see a plant that looks like it would be harvestable, we may not know what it is.