I'll elaborate a little on a few of the points (I think it's a very interesting topic).
1. I knew a lot of people here wouldn't like expanded playoffs, but it would cause fan interest to jump significantly. Smaller market teams would also be much more likely to re-sign their stars I think - and the star players would be more likely to want to re-sign with the smaller market team. I also think it counters some of the payroll disparity to a larger degree.
2. Generational talents like Harper aren't generally going to get gobs more under the current system anyway, simply because they have no real advantage (but a ton of risk) for not coming to an agreement if they are one of the first few picks. What this would do is make the whole process go much smoother, and allow teams to get players into their system immediately and give them an extra 1/2 year of development (which is good for the players too). I'm not sure more players going the college route is a bad thing at all. They probably get just as much playing experience and instruction (if not more) than they do in short season anyhow. It would also increase signing bonuses for a lot of later round college guys with no leverage who teams currently abuse to free up money.
3. I agree the players union would never agree to it, but I think it would improve the game. Guys would be more likely to get more money if they stayed healthy and productive, and less if they didn't which is how it really should be. It would also keep guys hungry, rather than seeming to just sit back and veg after they get their mega contracts.
4. The clause may not be necessary (can't recall exactly what I was concerned about when I added it). I like the rest though, as I think it would provide much more fair compensation over a much larger range of players.
5. Parity hasn't seemed as big a deal the last few years (and expanded playoffs would make it MUCH less of a concern imo), but in reality the firm cap would probably only affect the upper limit on 2 teams this year. In fact a higher floor might affect more teams next year than a lower ceiling would. I don't really agree that the difference in general is front office intelligence, but rather teams being forced to operate in that manner based on their payroll limits. I honestly think that if you gave Beane or Friedman a boatload more money that they'd make the same kind of moves other larger market teams make. They might be a little more successful at it (especially Beane), but I think the type of moves would be the same. The exception imo is the Cards front office, who I don't think would significantly change their habits.
6. The devil would certainly be in the details, but currently it seems that a lot of the load for new stadiums comes from a city's tax payers. This would shift the load specifically to baseball fans. Teams would no longer need to worry about stadium leases or debt, and all stadiums would be baseball stadiums rather than needing to be multi-use facilities. I don't think there'd be an increase in infighting for stadium slots. There could be a simple initial lottery, after which the slots continually rolled over. Alternatively, equal amounts could go into each teams stadium fund each year, and teams could choose when to pull that money out and have the stadium built once sufficient funds were built up.