How different are Jehovah's Witness beliefs?

To start at the end:  Not very different at all.

Charles Taze Russell, the founder of Jehovah's Witnesses, was originally a Presbyterian and quite a lot of JW doctrines are held in common with traditional Presbyterians.  Like a lot of strict Presbyterians, particularly in Scotland, the JWs don't like drinking or smoking and are dubious about dancing.  Both denominations believe that the Bible is the word of God and that salvation is needed to get God's reward in the afterlife.  And the hymns that JWs sing are just slightly re-worded versions of generally popular hymns.

Perhaps the best known doctrine of the JWs is that we live in the "end times":  That Armageddon, the end of the secular world, is around the corner.  And that is actually a common belief among a variety of Protestant groups, though usually a belief by a particular congregation rather than by the whole of a denomination.  Matthew 24 makes it pretty clear that Jesus too believed that the end was nigh, and the apostle Paul clearly did (1 Corinthians 15:51), so it is hardly surprising that some Christians still do.

And their pacifism is also shared by a variety of other Christians -- such as the Quakers.  After reading Matthew 5:39 I was once a pacifist myself.

Their rejection of blood transfusions is a little peculiar but it should be noted that both Jews and Muslims are superstitious about blood and take great care not to eat any.  JWs think likewise but add that it is inconsistent to avoid taking blood into your body via your mouth and then take it in by other means.  So their sensitivity there is just a refinement of a prohibition followed by over a billion people.

And here's the kicker about that:  There was a study of survival after heart surgery that took a particular interest in survival by JWs.  Apparently transfusions are common during  heart surgery so they expected a greater mortality among JWs after they had refused transfusions.  The study found that about a third of non-JWs died but NO JWs did.  Tranfusions cause stresses of their own.  Use of transfusions has declined markedly since then.  So JWs did have the last laugh.  God's wisdom?  They think so.

JWs also reject the messy doctrine of the Holy Trinity but they are not entirely alone in that.  As the name implies, Unitarians do too -- if there are any of those left.  Christadelphians also reject the Trinity doctrine.  But it is a major break from Chistianity generally.  Even Seventh day Adventists accept the Trinity.  It should be noted that the doctrine of the Trinity was introduced by Athanasius in the fourth century as a theological compromise. Even the word "Trinity" is not mentioned in the Bible.

JWs also reject Christmas and Easter as being pagan celebrations but that is widely acknowledged among more scholarly Christians.

But the biggest break from other denominations is the JW belief that the soul is not immortal.  Since there are quite a lot of places in the Bible where the soul is said to die, it is not a surprising belief but the doctrine of the immortal soul is apparently too ego-pleasing for anyone else to give up.  Since  the favorite scripture of most Protestants -- John 3:16 -- says you don't automatically get immortality -- you can perish -- it is a real wonder that the belief in an immortal soul is so widespread.  Ego trumps scripture.

It should be noted, however, that the original Jewish teaching was that eternal life for the righteous was attained by resurrection at the second coming of the Messiah. Popping off to heaven when you die was ignored as a pagan teaching.  Jews believe all sorts of things these days but most would, I think, be comfortable enough with JW teaching on the prospect of an afterlife. As the Jewish Encyclopedia says:

"The belief that the soul continues its existence after the dissolution of the body is a matter of philosophical or theological speculation rather than of simple faith, and is accordingly nowhere expressly taught in Holy Scripture"

So JW's have at least some Jews on their side in the matter.  Their view of the afterlife could be said to be Jewish.

On church government JWs departed early on from Presbyterian practice.  Pastor Russell was originally elected but that seems to have just faded out. JWs are governed by a central government, a theocracy, unlike the democratic Presbyterian practice.  JWs are governed much as Roman Catholics are -- but their "pope" (Don Alden Adams) keeps a low profile these days.

The best-known difference of JWs is their practice of doorstep preaching but the Mormons do that too.

The overall zeal of JWs  is striking.  Hitler gassed a lot of them for refusing to bow the knee to him. But such zeal has much precedent among other Christians.  Can you believe that at one stage even the Church of England had bishops being burnt at the stake for their faith?

So there is no major point of JW doctrine that is not held in common with some other Christians or Jews.  Like all other denominations, JW beliefs are a particular pick-and-mix of common beliefs.  It is probably true, however, that the particular pick-and-mix chosen by JWs is closer to first century Christianity than is the doctrine-set of any other denomination.