Scope of a Chart: Life, Not Psyche
One practical way in which medieval and modern astrologies differ - and where it starts to get fun - is that modern astrology tends to frame the natal chart as a reflection of the native's psyche while medieval astrology sees the chart as a reflection of the life's circumstances and events.
This means that modern astrology regards the chart as being descriptive of how the native feels about or responds to things. Within this context, the seventh house of relationships is descriptive of how the native feels about or operates in relationships, or perhaps what they seek to get out of relationships. In medieval astrology, the seventh house is descriptive of the partner or types of partners that the native gravitates towards or attracts. This internal/external split defines much of the differences between these types of astrology.
The houses themselves are a good source of evidence for this split, but so too are the planets.
This contextual shift is important. Viewing the chart as life events or the quality of those life events that surround an individual allows for a more concrete predictive technique. There's no need to second guess what is an internal feeling and what is an outward experience, because the lines are already drawn in the nativity. This also allows an individual to take a step back and not feel compelled to identify with every placement in the chart. Trying to do so often leads to feeling overwhelmed and confused, as the different placements may have incompatible, if not directly contradictory, interpretations. Through this methodology, the astrologer knows what parts of the chart belong with and speak for the native, and knows which of those that signify or control external circumstances.
Modern astrology tends towards giving everything subjective value while classical forms tend towards weighing everything as a conventional good or evil. Some things are good and we want to experience them (friends, love, health) while other things are bad and we don't want to experience them (isolation, sickness, anxiety). Classical astrology is prepared to help us understand what the manifestations of these things in any given life is likely to be like and when we can realistically expect them to happen.