I like the concept, except for the voice part. I think that is a wank feature, but to each his own. As long as I can turn it off I don't care. I think some carefully designed audible alerts would be more useful.
A few more thoughts: I am surprised that you balk at putting an opto-isolated USB input when you already have at least 4 isolation barriers. Assuming volts-ohms-cap-diode is on the main supply, the three extra channels plus the thermocouples will need isolation, both of signal and power. USB only needs signal unless you want to power/charge the unit via USB. Another factor related to the isolators is power consumption. The isolated sections will take more power, so it would be nice if "channel 1" could do all basic measurements with the isolators turned off. This would mean an extra amps jack, or for channel 1 volts/amps to share a common terminal.
Obviously the SD card is great for logging. It can be handy for firmware upgrades, but it can also be a bit fiddly -- it is an extra step, and it requires me to have an SD card and an SD reader handy.
A feature that I would really like on meters is unlimited custom ranges. A lot of bench meters come with a calibration for a PT100 or PT1000 that gives a direct readout of temperature, but what if I have a PT500 or a thermistor or an AD590? Transforming a current/voltage/resistance to an arbitrary unit would be really useful -- this could also be handy for monitoring power when you have a voltage proportional to current, for instance if you have a hall sensor or an external shunt like the uCurrent adapter. USB might be a handier way to load calibration curves on the meter. For volts and current, 90% of the time you can get away with a linear + offset scale which could be entered via the onboard buttons, but a thermistor needs to have a curve loaded.
With regard to a range switch: I am on the fence. The standard range switch is a bit difficult on a 4/6 channel meter. If you put one on, it will probably only select the operation of channel 1, or possibly include 'power' that uses primary volts and amps. You might be able to have a soft range switch based on a touch sensor or a rotary encoder (think the original iPod with the mechanical wheel). Then you could press the soft button associated with the channel you want to select and spin the wheel until it hits the right setting. Another soft button to select manual ranging, and spin the wheel to select range, and another soft button to select custom defined scale associated with that measurement type (volts, ohms, amps, diode), and spin the wheel to select it.
A complicated measurement is going to be a bit crazy with the test leads. In order to ease confusion and reduce the risk of error, I recommend thinking heavily about distinguishing the jacks. Maybe take a cue from the oscilloscope manufacturers. Have each signal channel (1-4) have their own color, and have the common terminal half black / half signal color.
I don't know how much is involved in the ohms/diode/capacitance/continuity measurement. If it isn't too complicated, I would put all the secondary features on the second voltage channel, even if they are harder to get to in the menu system. I can imagine that you have everything set up to measure power, and decide you want to monitor a resistor (maybe a thermistor) -- it would be nice if you could do that without changing all your probes around and resetting the meter to use channels 3 and 4 for power.