I took this as a poll about the game itself, excluding the priceless bonds formed between players and characters that I'm sure for many far outweigh all else, and are much influenced by the RP experience they've had over months or years.
I voted mechanics because at least from my perspective these spawn and support much of the rest. As a player, with perhaps limited creative IC dialog skills, I need to use the props and functional elements to support my RP and interactions with others. From sharing a bit of food and drink to trading in crafted things to being a valuable part of a party set on a task or adventure to just going off by myself to explore the lands and towns and enjoy the various NPC's (mobs and others alike), game mechanics for me are the support system that brings everything together. To even talk abut the weather it helps to actually have weather! Make everything work, and dress it up afterward I would say. Diversity and variety of things we can do and interact with, along with sensible balance among them, just seems to be at the core of the experience. Along with these, Story provides for me the backdrop and details that drive the RP and the purpose behind the things we have and activities we do. This for me is sufficiently important as to not be represented as second place, but rather runs near even with mechanics in support of all else.
Second choice would have to be graphics. The more diverse and immersive scenery, models, items, and effects the better. It doesn't need to be over-the top stuff either...the small details make so much difference. Things like how the rogues have that shoulder-roll idle action. Spells don't have to have big flashy effects, often simple is better, less is more.
MrG