Without access to the Recordings yet, some of these questions are difficult for me to answer with accuracy, as my answers will likely adjust after watching more of the video recordings.
What did you like about the content at TC14?
Wide variety, lots of excellent Customer-led sessions available.
What did you NOT like about the content at TC14?
The seat limitations (registration for sessions is definitely a necessity at this point). The time limitations on some presentations -- I believe Joe Mako or Jonathan Drummey could've presented on their topics for 2-3 hours. I'd attend those sessions in a heartbeat.
What kind of content do you want to see more of at TC15?
There seem to be plenty of content types; not sure I can suggest any more here. I dig the sessions that get into helpful solutions to a variety of problems. The Tableau doctor idea is nice, but I'd like to have people outside of Tableau available for help as well, if they are willing--and I have a feeling some are willing. (I'll feel very ignorant if some of the Tableau doctors are non-Tableau employees).
What do you want to see more of from customer speakers at TC15?
More time/more seats given to their presentations. Provide registration in advance, and adjust seats based on demand.
What do you want to see more of from Tableau speakers at TC15?
More real-world datasets (with NULL values, etc.) in use/more problem workarounds explored. I did quite enjoy seeing several Customer AND Tableau speakers referring to different paths to different solutions, based on a user's dataset and end goals. Keep that going; it helps a wide range of folks.
Any other content suggestions you have as we gear up to take over Vegas!
Perhaps provide alternatives to seats in a session--cue up high-demand sessions in a lounge area with monitors set up, as was done for the Keynote presentations this year.
I'm sure I could come up with more/different feedback tomorrow, but I hope this helps.