I would like to propose some options which users could use to "limit" or "cripple" their system, at least in comparison to the usual features available. Some options only apply to the patches included in the non-vanilla releases. Here are some examples :

machine=mda, tandy_old, mcga
mda - Emulation of IBM's Monochrome/Display Adapter, no Hercules support.
mcga - Emulation of IBM's Multicolor Graphics Array, no VGA features and only 64KB of video memory allowed. Also may require PS/2 BIOS identifier bytes.
tandy_old - Emulation of the pre-TL/SL chip, no 640x200x16 mode available.

monitormode=color, color200, mono
This would allow EGA, MCGA and VGA machine types to give the appropriate behavior when connected to a monochrome monitor. Modes 7 & F would function correctly in these modes for EGA and VGA. Non-functional for other machine types. color200 is for EGA only and tells the software that 350-line color modes and 64-color palettes are not available (as if the EGA were connected to a 200-line RGBI display).

egabase=3xx, 2xx
This allows the EGA card to be used at the alternative I/O addresses, but this is admittedly really out there.

tandy=on_notandyvideo
This allows the Tandy to work with a non-Tandy video machine type. Essentially the sound chip and/or DAC would be available, the BIOS would identify itself as a Tandy 1000, but the built-in video would be disabled.
tandybase=C0, 1E0
This latter address allows full use of a 2nd DMA controller, as seen in the 1000RSX, 2500XL and Sensation machines.
tandydac=true, false
This allows the user not have the functionality of the Tandy DAC. Games will have to use some other device for sound effects.

innovaclock=actual, ntsc, pal
This allows the user to control the clock rate to the 6581 SID in the Innovation SSI-2001. NTSC gives 1.02MHz, PAL .985MHz and actual is .895MHz.

gusram=256, 512, 768, 1024 (or 0, 1, 2, 3)
Controls the amount of RAM available to the Gravis Ultrasound.

sbtype=adlib, adlib_gb
adlib - Only an OPL2 chip is available at 388/389H, no sound blaster features.
adlib_gb-Gameblaster available at 2x0, Adlib at 388/389, still no sound blaster features.

mididevice=mt32old, mt32new, cm32l, la_userdefine, cm
The mt32new would require a 2.xx control ROM, and have no support for the 33 sound effects of a CM-32L, LAPC-I etc. The mt32old would emulate the quirks of the older 1.xx Control ROM and hardware, the 40ms sysex delay, and the loudness cutoff.

The la_userdefine (as in Roland LA synthesis) allows the user to essentially pick and choose which features and drawbacks, to the extent they are not mutually incompatible would be supported as shown in the current MUNT implementation in DOSBox.

egamemsize=64,128,192,256
Only has effect with the EGA machine type, this will tell the emulator to allow access to less than the ideal amount of RAM.

tandyvideo=lower, upper
This tells the system whether there is 128K of video memory at the 640-768K area of the memory map, or whether Tandy video starts at the end of usual conventional memory area (640K or lower). This can be combined with the memsizekb function to cut the available conventional memory available in the Tandy mode even further.

cputype=8086, V30, 286
This is designed to limit the processor features available to the program. A more ambitious feature to be sure, essentially this would eliminate the ability to use opcodes and memory modes of the 386 and higher processors. In these modes, UMBs should not be available, EMS only via emsboard, and XMS only on the 286.

This ends the examples. The idea is to defeat game routines that will give better graphics or sound capabilities than you actually want. Primarily for testing and pure nostalgia purposes.