It is becoming pretty common to use a few more cards in your system than only a graphics card. A gaming system without a modem, ISDN or network card is certainly not worth being called a gaming system anymore, simply due to the fact that the only real gaming experience is generated by multiplayer games, my beloved Quake II is only one of many many others. Hence it's not out of the world if I expect that any network card should work flawlessly in any motherboard.
People who buy expensive Pentium II systems are certainly making a smart move when investing in SCSI rather than EIDE. SCSI still offers the highest disk performance, a great upgradeability for e.g. CDROMs, CD-recorders, scanners, streamers, ... and last but not least a very low trouble level. Thus I do appreciate if motherboards that are targeted towards expensive high end systems have got a SCSI adapter already onboard, a RAIDport is even better, and it's almost perfect if it's even Adaptec's latest U2W SCSI adapter, as e.g. on DFI's new BX board. The least I would expect however, is that any SCSI adapter runs flawlessly in any board.
A sound system is nowadays a basic component of any PC. Thus I'd appreciate if there's either a decent sound system onboard or the board works fine with older ISA soundcards as well as the new PCI soundcards. In case of the latter it's useful having the new 'SBLink' onboard, which enables compatibility to the old ISA Soundblaster standard.
All in all do I think it's not really asked too much that a modern motherboard can host all these components together at the same time. If it doesn't, it may be as fast as it wants, it will still be pretty useless for any home or office user, system integrator or OEM.
Another requirement of a motherboard is certainly the stability. In the most cases boards become instable when they cannot work properly with the RAM that's plugged in. As we are fast moving towards the 100 MHz system bus as a standard, memory problems will become a lot more common. It can easily be that a board only works reliably with RAM of only a few memory vendors, other boards were designed and tested better, so that you can throw virtually any memory at it, as long as it applies to the basic specifcations.
One way of testing this out is of course overclocking. If the board is running stable at a higher system bus than what it was designed for, it will most likely be rock stable at the specified system clock. However, testing a board to the limits is very difficult, because no board manufacturer and neither any CPU manufacturer would tell you which instructions are most sensitive to timing problems and overclocking.
So it's virtually impossible saying that a board or a CPU run absolutely stable at a particular clock speed, because it is very likely that the really touchy procedures haven't been ran at all. This means for the reader that you of course can be lucky as long as you are not using these procedures on your system, but it could as well be that you are using particularly the very software that will cause a crash in a board that was testified as stable.