This may be a silly question, but what exactly did Jez focus on? Was he implying that I needed pull-downs for the off state? or what?

Hallo George,

at input of 74HCT540 switches are connected, right? Switches having only two contacts need an additional resistor for working correctly. Connect eight 10k pull-up resistors to inputs of 74HCT540, one terminal to input, the other terminal to Vcc. Then, connect each switch with one terminal to input of 74HCT540 and with the other terminal to ground. If your switch makes a connection between both terminals when be pressed, low state is fabricated at according input of 74HCT540. If switch is not pressed, due to existing pull-up resistor high state is fabricated at according input of 74HCT540.
By the way, if you use this pull-up scheme, you can use 74HC540 instead of 74HCT540. There are some small advantages.

I think, Jez was missing these pull-up resistors in your schematic.

As for the RAM, that was just a silly oversight. What would be the standard way to control the Chip Enable? An AND of the !RD and !WR lines?

This could be one solution. Have a look at datasheet of your RAM, in any case. If your RAM is a fast one, I see no problems. In former times, one had to keep in mind, that data access time from !CE becoming active was some bigger than data access time from !RD becoming active. But !RD low pulse of mcu should be truely large enough for your RAM.
Some RAMs show a significant increase in supply current, when being enabled. In order to prevent trouble, connect a 100nF X7R capacitor as close as possible to power supply terminals of RAM.

The reason for the resistor between the supervisor and the reset line is due to a peculiarity with the 89c51 - for the watchdog to function correctly, there must be at lease a 2k resitance between reset and any external circuitry.

Nevertheless I highly recommend the use of additional 4k7...10k pull-up resistor at reset output pin of MAX1232. Voltage at reset input pin of mcu must track Vcc at power-up, even from very low voltages on. I remember an errata sheet, where this additional pull-up was recommended for trouble shooting.

Bye,
Kai