I think Rabbi Kaii has provided this an excellent answer. He has given the broad strokes, and I would emphasize that the experience of many Orthodox Jews would be that they would feel "at home" in an Orthodox/traditional Shabbat service in any country in the world, even across the Ashkenazi/Sefardi divide, but going to a Reform synagogue across town would be disorienting and extremely difficult halachically, particularly because of the following:
(1) Reform congregations often use microphones and musical instruments, which Orthodox Jews would see as strict violations of Shabbat laws, and even worse, these violations are occurring publicly and intentionally, which carry strict penalties under Torah law.
(2) Many prayers are skipped and/or recited in English and/or altered, which means that Orthodox Jews would not fulfill their obligation (or feel they did not fulfill their obligation) by attending these services unless they can find a traditional prayer book.
(3) The use of female cantors (who lead the congregation in prayer and in reading the Torah), and also females called up to the Torah, as well as mixed seating, are alien to Orthodox Judaism.
On the flip side, I know from experience that many Reform Jews are quite uncomfortable at an Orthodox service. For one thing, unless it is a bar mitzvah and the Rabbi sees that there are many non-traditional Jews present, it is rare that congregants are told which page/prayer they are on, and much of the service is davened (prayed) in an undertone. As a result, many Reform Jews will not know what's going on, and will need someone next to them to show them "where we are." And there will be very many prayers with which they will be unfamiliar. There are of course exceptions, and some will be able to follow along well, but that would be the minority.
Second, Reform Jews, who are used to egalitarianism, which in fact is one of the focal points of Reform Judaism, will be struck by the lack of female involvement in any aspect of the service. I have heard from many Reform Jews (male and female) that the reason they avoid Orthodox synagogues is because they palpably feel a sense of injustice during the service due to what they view as gender inequality. The same issue may arise for Conservative Jews, who are generally trending in that direction.
So for very concrete reasons, the divide between the the more liberal and traditional forms of Judaism results in a situation where both sides would feel a great deal of discomfort attending a service of the other. This is really unfortunate, and I wish I knew where the solution lies.