There's some pretty simple stuff that can be done to help people get into the game (some already mentioned).
1- Bundle a pregenerated world with a few 'example' forts at various stages so players can get into the game right away seeing what it actually looks like when a fort is going along).
2- I'd scrap the 'play now' option entirely. Instead provide some starting profiles and have them actually make sense. The Play Now is troublesome not only because it's not a very good starting mix but it also doesn't show players what a starting mix looks like.
3- Possibly provide some overall option to find a site... where newbies can click one button and find an 'easy' site. Something like peaceful, trees, no aquifer, flux stone, etc.
4- Have [VARIED_GROUND_TILES:NO] the default option. One of the most confusing things about playing Dwarf fortress in no graphics mode as a newbie is the 'tile vomit' of varied ground (and stone, see below). When you first start playing and see something like this
http://www.indiegames.com/blog/images/timw/dwarffor3a.png it's just a lot of visual noise that you have no idea what it is. The varied ground makes it a lot harder to learn all of the important symbols.
5- Group rock/ore/gem icons better. All flux stones, metal ores, Adamantine, obsidian, bauxite, bituminous coal, and gems should have 1 single icon per group and just vary in colors. Soil should all be one icon (I think it mostly is anyways). For 'normal' stone have them have icons based on type (like sedimentary) and occurrence (clusters vs veins vs layer). As a new player you spend a lot of time just trying to figure out what each rock is and if its useful to you.
6- A better 'tech tree' on the website or in help. Possibly a list of recipes. The dwarf fort wiki is good for this but having a single cheat sheet would be helpful. Just things like what type of processing each plant can go through, growing requirements, building requirements (ie anvil for metalsmith forge, 3 bars for platemail, etc.) and a quick reference of where to get items (e.g Cloth from Loom, Charcoal from Wood Furnace).
7- An F.A.Q or quick guide somewhere a little more official. Mostly just explaining some simple tasks that are a little buried in the UI like how you need to enable/disable labors for your dwarves if you want more things done.