There are two polls. This was the MUST poll:
http://www.enworld.org/showthread.php?t=205162&page=2
I did the poll in part to highlight that D&D constits of a lot more then the very particular mechanics or story elements that people (including myself) obsess on.
Here is the "top 40" chosen by those who bothered to answer (% is fraction who say it must be in game):
1) d20s 90.85%
2) Spells 90.14%
3) Dungeon adventuring 88.73%
4) Levels (for class/character) 87.32%
5) dragons 86.62%
6) RACES 85.56%
7) Magic Items 85.21%
8) human 85.21%
9) MONSTERS 85.21%
10) CLASSES 83.80%
11) Wilderness adventuring 80.99%
12) elf 80.99%
13) dwarf 80.99%
14) fighter 79.58%
15) goblins (and kin) 79.58%
16) wizard/magic-user 78.87%
17) orcs 78.17%
18) rogue/thief 77.46%
19) giants 77.46%
20) Six abilities 77.11%
21) eXPerience points 77.11%
22) d4s, d6s, d8s, d10s, d12s, d00s 76.41%
23) City adventuring 76.06%
24) HP 75.70%
25) cleric 75.70%
26) trolls 75.35%
27) Gems and Gold 74.30%
28) demons and devils 72.89%
29) evil humans 70.77%
30) kobolds 69.72%
31) elementals (and kin) 69.72%
32) beholders 69.37%
33) mind-flayers 67.25%
34) AC (damage avoidance) 66.55%
35) griffons and minators (and so on) 65.49%
36) halfling 63.73%
37) Multi-Classing 63.38%
38) Saving Throws 61.27%
39) fey 58.80%
40) 20 or more kinds of weapons 57.04%