Great summary of TESOL France - thanks for sharing it. I've both attended and organized more conferences than I can remember. Here are my thoughts - in no particular order:
1. Consider having streams around topics, such as technology, listening or assessment, etc. and/or around teaching contexts, such as ESL Literacy, EAP, Workplace English, Working with Youth, etc. so that people can follow the stream across the conference. This usually requires some fancy scheduling to ensure options at each or at least a number of time slot and little/no duplication.
2. I like a 2-day conference with a pre-conference symposium day (with 3 hour sessions). During the conference, if possible, a mix of 60 and 90 minute sessions is nice - sessions so often start late that 60 minutes is the absolute least you can do as it will probably just be 45 minutes in reality).
3. I like 1 plenary a day. More than that and there is little time for concurrent sessions - I DISLIKE when there are concurrents at the same time as plenaries.
4. I like it when a lunch is provided at least on one day. It's a nice time to chat with folks. I find banquets and the usual entertainment boring and prefer to go out with friends and explore the town. However, I always enjoy a wine & cheese. Coffee and nutritional snacks at breaks is imperative - and let's have fewer pastries. Sugary carbs just make participants snooze through the next sessions.
5. I often do multiple sessions at conferences so have little time to attend workshops; however, I like to pop into a session or 2 if I can, so I appreciate not having to register.
6. A good publishers' display is fundamental to a good conference.
Thanks for this chance to contribute!