I doubt these are going to be the most popular opinions but here goes:
1. Don't include any optional or advanced rules. Either it is part of the game, or it isn't. If you want to post up some optional stuff on the MUL or the BT site that's fine, but don't put it into the 'official' rulebook.
2. No construction rules/PV Calculations. Put that information on the MUL, so it can be easily and centrally updated. Publishing in a book makes it too annoying to change.
3. Leave out pure aerospace combat. Aero on the ground map is OK, but the radar and capital map side of things...I don't think they are really in a good place in terms of being fun and balanced, so I'd keep them out of the printed book for now.
Regarding what the other posters mentioned, I agree with the 'one book to rule them all' concept. Roll everything that is going to continue on into one book instead of two. Part of AS being the faster, simpler option for playing BattleTech could be keeping to one core rulebook.
I'm not a fan of having woods not count if you are at the edge, or having woods reduce your TMM. I have my reasons, which I will avoid laying out so this post doesn't get any longer, but the bottom line is I don't think those changes make the game better, they just make it different.