I'd say alignment is pretty iconic to D&D myself. They make memes about it. I'd venture out to the general public because these forums represent the vocal minority. That minority has a lot more power now than it did even 5 years ago. Let's see what the public really has to say?
First: I mean no offense, but there's legitimately no way that we could get a "general public" poll that would mean a dang thing. You'd need an actual, professional polling agency, you'd need to completely redesign the poll to avoid various sources of unintentional bias, and you'd need an active sampling method that got a genuinely representative sample of D&D players, which is going to be extremely hard to do. These polls, and the proposed Reddit stuff, are literally the best we will ever get our hands on, because it's unlikely that
even WotC itself can do all that much better, apart from total number of people spoken to. I doubt WotC has conducted a single truly serious poll of their D&D fans in the past decade. (The Magic team handles these things
quite differently, from what I've seen, with the whole psychographic profiles and all.)
Second: "Iconic" =/= "needed for the feel," and (even moreso) "iconic" =/= "should be retained." THAC0 was iconic for 2nd edition, but emphatically wasn't needed to
feel like you were playing D&D. I completely agree that it's very easy to identify a game as being D&D-related if you know you've got a "Lawful Good Wizard," but even knowing that you are playing a "Lawful Good Wizard" does not in and of itself make players feel like they're getting the D&D experience.
For comparison, bald eagles are
iconic of the United States, but aren't necessary for feeling patriotic for the USA. Evoking or specifically depicting a bald eagle is a great
shorthand for signalling "you should associate this thing with the United States," but it's no guarantee that it will rouse patriotic feelings in the hearts of onlookers. The Stars and Stripes, on the other hand, is both iconic
and inherently patriotic-feeling, for a variety of reasons (flags having long been explicitly identified with the nation they represent--hence why dirtying or burning a flag tends to inflame tempers). Conversely, the Lincoln Memorial is quite patriotic, but unless you are
directly looking at the statue of Lincoln, the building is terribly unassuming from the exterior. Don't get me wrong, it's a very beautiful building that I'd love to see in person some day, and for us here in the States it is likely to evoke strong emotion because we recognize it, but for someone from outside the US, it just looks like any other marble-facade government building (unlike, say, the Washington Monument or US Capitol Building, which have more distinctive shape).
Alignment is iconic, emblematic, but not high on most folks' "you need it for it to feel like D&D" list, nor their "must be retained" list.