I think a lot of posters have already covered most everything I could think of, so I'd just like to add my support primarily for marketing/trade and system data, but also as much data of every kind as possible. When it comes to raw data, more is better 
Wishlist:
As for the uses of the data, I am interested in developing my own data analysis tools, primarily for friends, but also for the broader community if they proved to be useful. I've already been using Google Spreadsheets to compile, filter, and prioritise market data, as well as catalog some cartography data too. I see the potential for some very sweet, functional apps/web apps developed by the community off the back of it, be it market trending, stock tickers, galactic control maps, cartography stats, and so forth. Not knowing the full extent of Frontier's plans for the future of ED, it's possible that some of this will be catered for directly anyway in due course?
(Next part in spoiler tags in case it is considered discussing merits)
Wishlist:
- Market commodity data (buy/sell/supply/demand) on a per-station basis - limited to only the stations I have visited recently, if necessary, though I see how this information could be pooled through community efforts
- Station/planet/system data (allegiances/governments/economies/system factions and their current status)
- Ship/outfitting data (ship types/prices/hardpoints/internal/module types/data/availability)
- System traffic data (like in the newsfeed)
- Bounty data (like in newsfeed, but maybe also some kind of most wanted list?)
- Pilot data (allegiances/standings on per-system basis and global/ranks/military ranks/finances/query other pilots)
As for the uses of the data, I am interested in developing my own data analysis tools, primarily for friends, but also for the broader community if they proved to be useful. I've already been using Google Spreadsheets to compile, filter, and prioritise market data, as well as catalog some cartography data too. I see the potential for some very sweet, functional apps/web apps developed by the community off the back of it, be it market trending, stock tickers, galactic control maps, cartography stats, and so forth. Not knowing the full extent of Frontier's plans for the future of ED, it's possible that some of this will be catered for directly anyway in due course?
(Next part in spoiler tags in case it is considered discussing merits)
Whether or not you choose to provide the data to players in a convenient form such as an API, they will always have ways of gathering, pooling, and processing the data -- be it through OCR, or manually typing figures into a spreadsheet. That's part of human nature. The downside of this is that it takes players out from the game and forces them into technical or otherwise tedious workflows, just so they can maximise their time and efficiency when they do play, breaking the immersion and losing valuable play time. If the functionality isn't incorporated directly into the game, then making it as easy as possible for community developers and, by proxy, players seems like the next best thing. Less hoop jumping, more playing!
I think the best companies out there are the ones who open up with and embrace the community spirit of collaboration and external development
I think the best companies out there are the ones who open up with and embrace the community spirit of collaboration and external development