Squares between any of the five mutual placements are definitely difficult and indicate an element of significant incompatibility, but the compatibility has to be taken as whole, and a little bit of square energy can be good. When the aspects are formed between whole signs, but not by actual degrees, they are important, but nowhere near as significant as an actual aspect. Another basic compatibility test is to see if two people are basically of the same gender. Masculine people need masculine partners. Feminine people need feminine partners.
Balanced people can go both ways, but are best with balanced partners.
The more similar two people's astrological gender is, the more compatible they will be. Conjunctions and oppositions between the Moon and the Ascendant, or the Sun and the Moon are some of the best aspects for long-term compatibility. Conjunctions, trines, and oppositions between Venus and Venus, Venus and Mars, or Mars and Mars are the best for sexual desire and compatibility. Some stressful energy is good for a relationship in that it gives the couple an impetus to grow together, and provides polarity, which equals passion. The Moon is generally considered the most important 'planet' in relationships.