Polarity protection
If you look at the LM317 specs, there are two diode that can be added for reverse polarity protection. These prvent destruction of the regulator if an external source is connected to the power supply output.
+ and - outputs from the same knob is very unusual, however I have run into a few HP models that did that. They happened to be called a power supply/amplifier. They were about 3 Amps at 60 V or so, but only had a few KHz bandwidth.
Something that hasn't been mentioned is 4-quadrant operation. The above had that feature. 4-quadrant means it can operate +V and +I, -V and -I and -V and +I and +V and -I modes.
Goto
www.circuitspecialists.com and type "electronic load" into the search box.
One of the more ultimate power supplies is the source measure unit.
http://www.home.agilent.com/agilent/product.jspx?nid=-33504.0.00&cc=US&lc=eng and
www.keithley.com make them.
Agilent's unit is capable of graphing a current-voltage cure on the front panel.
Series Operation of identical supplies allows one control to modify both outputs. e.g. One setpoint for voltage and 1 for current.
Parallel operation of identical supplies allows the current to be doubled.
Programming: Resistance, front panel, voltage, USB and IEEE-488 are also possible.
Isolated analog programmability is almost essential because the setpoint voltage is usually applied relative to the positive terminal.
The ability to program lots of supplies from a single interface helps in ATE situations and lots of times the analog programming is more useful than the digital programming.
Isolated analog out is yet another feature in some supplies.
A contact closure that shuts down the power supply is another useful feature. Sometimes it uses a special breaker which can be tripped with a voltage.
The fancier supplies allow a universal input 90-240 or 90-280 VAC input.
What was stated earlier was form factor or front panel space. I totally appreciated the ability to get a 1500 W power supply in a 1 Rack unit form factor and universal input.
Remote sense is mandatory for high current supplies.
Crowbar for Overvoltage protection
This gives you an idea of a nice supply.
https://www.electro-tech-online.com/custompdfs/2012/05/XG_1700_Datasheet.pdf
Note that there is the possibility to drive polarity reversal relays and output disconnect relays.
In my application, these were not available at the time and if they had a constant power mode they would have been more perfect.
One thing that you don't generally see on a power supply is the power in Watts as a metered value. In my application, this would have been useful as well.
LG mentioned the ability to set specific voltages such as 0.8, 1.8, 3.3, 5, 12, 15, 24 and custom could be useful.
So, I've been discussing possible options. It's always nice to start from an unobtainable list and then select. I encourage those to initially discus possible options. Later a vote migh decide on the "best" options.