There is the possibility that one or both of you will put the other on a pedestal and will be disappointed if that other turns out to be only human after all . In general you'll have difficulty seeing each other clearly and will be more likely to be gullible with each other. Because Neptune spends so many years in each sign, everyone born within a few years of each other will have Neptune in more or less the same position. If the aspect is within a degree or so of exact, it can indicate confusion in dealing with the relationship and with the desires of the people involved. If the aspect is within a degree or so of exact, it can show that this is an intense and energetic relationship in which each of you spurs the other on to achieve more and to work to improve your situation in life.
Close aspects are also a strong indicator of powerful sexual attraction.
This square is typically an indication of attraction, although there may be some friction. Yet, this friction is usually mild and may only add to the sexual tension between partners, which ain't all that bad. The Sun person's assertiveness may occasionally be felt as pushiness by the Venus person and the Venus person may sometimes seem oversensitive to the Sun person. It's not surprising with this aspect in your synastry comparison. There is likely to be a good deal of competitiveness, challenge and conflict when Mars opposes the Sun.