Since it seems that many people are a bit confused about this topic, I hope you guys do not mind if I sum up the good responses posted so far and also add some of my own believes and experience on the discussed topics.
Large speakers in small room problems:
1) Amount of bass. Large speakers (with large drivers) can easily overpower a small rooms by producing too much bass. Since most speakers do not allow one to adjust the amount of bass, tuning the bass output of a speaker for a given room can be extremely difficult (often impossible).
2) Speaker placement. Speaker placement in a room is crucial not only for stereo image and sound stage but also spectra balance. Placing a large speaker into a small room will limit considerably the ability to move the speaker around in order to minimize the speaker-room interaction, e.g. reflection, resonances.
3) Integration of the drivers. Large speakers with many drivers require one to sit farther away from them for perfect integration. Depending on the situation this may be impossible to achieve with large speakers in a small room.
Floor-standers vs. monitors + subwoofer
1) Volume level. Except for very expensive models, most floor-standers need to be cracked up quite a bit in order to get decent bass out of them. Often this is needed also in order to get the midrange to open up. Thus, IMO many floor-standers do not performed well at low volume levels. On the other hand, monitors excel here.
2) Speaker-room interaction. Monitors interact less with a room as they have a) smaller and more rigid cabinet walls, b) it is easier to place them in a room in such a way to minimize reflections, c) do not produce that much bass. Further, having the ability to move the bass source (the subwoofer) around the room is another huge advantage as almost always the speaker position that yields the best midrange integration and spectra balance differs from the position that yields the best bass integration in a room. Furthermore, most modern subwoofers provide additional adjustments, e.g. parametric equalizer, phase, crossover point, etc, that further aids the integration of the speaker system in the room.
3) Quality of low bass. Most decent subwoofer will have much more accurate and healthy bass than most good and very good floor-standers.
4) Mid-bass. Many argue here that most decent floor-standers get this region right while even expensive monitor - subwoofer combinations fail to do so.
Best wishes,
Paul