What are the main differences between 7th Day Adventists and Catholic/Protestant churches? What are the main differences between 7th Day Adventists and Catholic/Protestant churches?
The most notable position of the Adventist is the degree of Biblical Literalism. Generally, if the verses can have a literal meaning then that is the belief that they take.

The outsider will notice some or all of these if they spend a short time with an Adventist:


Worship on Saturday (after all, it is in their name. I will expand on this later).
Not eating unclean animals, per the OT Law distinction. That means no pork, like many Jews. The animal must have a cloven hoof and chew the cud, or be a fish with scales, or a non-predatory bird of the air.
No Alcohol or any other recreational drugs including caffeine. Medication is fine when necessary. I suspect they are hesitant of consuming opiates and other pain medications.
General emphasis on health and what you eat, although not doctrine, many are vegetarians.
No jewelry, save a simple wedding band, or excessively fancy clothes.


All of the above have Biblical justification, although those same verses are either ignored or interpreted differently in other denominations.

The outsider will likely not notice these unless they sit in on Adventist worship services and sermons or read Adventist literature.


Intense disdain for Sunday Worship. According to theology it is the mark of the beast (I will explain this later).
Intense disdain for the Roman Catholic Church (RCC). That is the Harlot Church of Revelation.
Intense disdain for the Papacy, not the Pope specifically. The office of the Papacy is the Antichrist.
High focus on eschatology and the second coming of Christ. "Keeping the eye on the prize" or "Finishing the race strong."
Intense guarding of the Young Earth Creationist view (YEC). They will argue this until they are blue in the face.
No special interest in a person Ellen G. White, but rather in her many writings and visions, most of which are regarded as prophetic or otherwise inspired.
There is no eternal quality of any kind to humanity. There is no such thing as the eternal soul. They are Annihilationists. 
The archangel called Michael is another name for Jesus.


I will address each one of these, now, referring to their number when necessary.

The first four are all highly related. That theology is also not new with the Adventists. The idea that the RCC is the harlot church (2) and its leader, the Pope, is the Antichrist  has its roots all the way back to the Reformation. The primary distinction concerning the Pope and the RCC between Reformation theology and Adventists is that Adventists call the Papacy, not the Pope (3), the AntiChrist (apparently, calling the Pope the Antichrist was a bit of a name calling thing half the time during the Reformation). What I believe is unique to them is that Sunday worship is the mark of the beast (the Antichrist). I first heard of it when studying Adventistism and have never heard it anywhere else. There is much theology around this, and really could be no fewer than three questions to cover it fully but here is the basics:


The Pope is called the "vicar of Christ, His voice on Earth." Antichrist is actually better translated as Psuedochrist. The Papacy is a fake Christ; he is not Christ's voice on Earth.
Naming the RCC as the Harlot Church is the same as naming it as the terrible beast from Daniel 7. To them, the RCC is the only "nation" that fits the description. The little horn that supplants three bigger horns is the Papacy. They match all this up very well with history and make a very sound argument.
The RCC changed Sabbath to Sunday. There is a lot of History to support that the RCC throughout history encouraged and even forced Sunday rest, instead of Sabbath rest. There is Catholic documents verifying this. With all of this in mind, plus a revelation by E. G. White which I will discuss later, leads them to conclude that calling and observing Sunday as the Sabbath is the mark of the beast, the RCC.


Now to YEC proponents and E. G. White (5 and 6). Ellen White is considered a prophet by Adventists. She wrote thousands of pages and is purported to have had many visions. Wikipedia article on her is extensive. One particular vision she had took her through the entire Creation. She witnessed it happen in 6 literal days and the climax was the Sabbath. Because God created everything in 6 literal days and rested on the seventh He sanctified the seventh day. Therefore, Adventists take the Sabbath very literally. After all, it is one of the commandments.

For point 7, the lack of an eternal soul, there is much to say and this could also be a separate question. They are not the only group to say this nor are they the only group to profess it today. The primary points that they use to support this are that in Genesis the word translated soul can also be translated as living creature or just plainly creature. It is the same word used for the animals that were created earlier. Also, no where in the Bible do the words "immortal soul" appear together. If that weren't enough the Bible says in more than one place that only God is immortal.

New section: I am always learning, it seems and I only recently learned that a large chunk of Adventists believe that the Archangel called Michael is Jesus (Point 8). Unlike the Jehovah's Witnesses, in the Adventist's mind this does not reduce the character of Jesus to a mere angel. It increases the character of Michael to the divine and holy Christ (Source). So when reading passages that mention Michael, Adventist's read Jesus, the begotten Son of God. To them, it is a lesser issue not worth "dwell[ing] upon at length." This is mostly because they way they interpret the scriptures redefines Michael, not Jesus; Jesus is still God.

For other Christian beliefs, they generally agree with the mainstream. The Trinity, Christ's divinity, holiness and infallibility of Scripture, etc.

The only group that is similar to Adventists is Jehovah's Witnesses, but only on the smaller issues like jewelry, save Annihilationism, which both groups hold. The Witnesses do also believe that Michael is Jesus, but in the opposite direction of Adventists; they believe that Jesus is lesser than God and they use the Michael is Jesus arguments to support that. Both Charles Taz Russell and E. G. White were influenced by Millerite Christians. After that, I would call them, and their theology quite unique.

There might be the inclination for outsiders to call them cultic or just plainly a cult, however, having studies with them (though I am not one), that is far from the truth. They are kind and understanding people, never resorting to trickery typical of a cult, very willing to help fellow Christians, whether Adventists or not, and even non-Christians, have a heart for mission work and to bring the gospel to all who will hear it, and are passionate about God, the Bible, Christ, and finally meeting Him at the end. They are a great bunch of people (the ones I have met anyway) and are worth knowing even if you do not buy the theology.