OK, some good things here, some already in the program.
I agree with the cleared session data. Right now if you 'clear' the session data, the numbers default to the target and the cell is highlighted. I like the highlight, as it tells me at a glance what numbers I have not entered from my brew sheet into the computer.
The program already has an alert if you exceed volume limits. On the 'mash' tab, there is a check on the volumes and the program will flag the 'mash tun volume needed' check by changing the dot from green to red. While this doesn't jump right out and slap the user in the face, it is there and should be a pre-brew day check. [I've done process modeling before and rule #1 is never trust the model even if proven.]
As far as the clearing session data clearing the notes, I put brewing and fermenting notes specific to the recipe in the 'notes' section. Clearing that would take out the notes needed to consistently brew some of my beers and force me to look up my written notes every time. Taste rating and tasting notes I would have no issue clearing, as I start fresh each session from a master copy of the recipe anyway.
The front end is the biggest issue people have complained about the program. While not super smooth and effortlessly intuitive, it is understandable and with practice becomes easier to use [note: easier, not easy]. I have yet to use another program out there that is entirely intuitive, as most people have their own ideas on what is a natural flow of information. In the end, I am pragmatic about the interface in that I want the program to deliver consistent results and be pretty flexible. On the list of things that need to be worked on, the interface is pretty much near the middle of the pack.
All-in-all, good suggestions to think about.