You can’t, beyond a few basic things, say that *all* Buddhists believe something, since Buddhism is like all of philosophy reinvented in its own image. There are sooo many Buddhist philosophers; Nagarjuna is just the greatest one. And Nagarjuna is very important to all Buddhists except for the ones in India/Sri Lanka and some of Cambodia.

*On top of that* you have religious Buddhism and philosophical Buddhism. Supposedly, the Buddha made fun of his followers for supposing that he was immortal, but then you get other sources saying that he was uniquely divine. I guess you rarely start anything spiritual in premodern times without being deified at least a bit. Even Nagarjuna was deified as being able to speak with snake-gods, and he is pretty rational.

Karma — In Hinduism, karma is like a cosmic accountancy system, governing the cycle of birth and death. This is what most We11sterners mean when they refer to karma. Some Buddhist texts are like this too, but in Buddhism, there is no caste system. Often, when the Buddha talks about karma, he means something more akin to “activity” or “causation.” Not in the sense of Western / Aristotelian ideas, but more like a descriptor of why the world (as he sees it) becomes more and more uneven or differentiated. Karma is not so much specific actions, which give you plus and minus points, as in Hinduism, but more like accumulated causation up to this point (to pre-Nagarjuna Buddhists). So, for example, for a European girl to have blond hair, there would have to be a massive accumulation of prior karma in the universe (big bang, apes, humans, Northern Europeans, etc.) that come together to determine that. It’s like a huge backlog of causation, to which you can add your own little speck. Because everything you do (and even think) has some kind of karma as well, adding to the molehill.

This is roughly what the Buddha taught. In religious Buddhism this then becomes construed to something more like what Hinduism was; — if you do the “good” things you will get a good “result” and have an easier time reaching nirvana. But the Buddha denied this, as did several famous pre-Nagarjuna monks. It was more of a folk belief.

Nagarjuna, of course, doesn’t need karma. He does not deny or affirm it; it’s is just empty of self-existed, co-caused by the universe and the universe co-caused by it..

Reincarnation — Depends on the branch and on philosophical/religious
Buddhism. People often ask: “If there’s no self in Buddhism, what is
reborn?” To pre-Nagarjuna (philosophical) Buddhists, what’s “reborn”
is actually the karma of the things you did — your little specks of
causation you added to the molehill, even though “you” are not there
anymore. A good pointer against the religious belief that you are
somehow reborn as something higher or closer to nirvana is the
Buddha’s own assertion that he was not immortal and would not be
reborn. But people believe what they will — a Thai Buddhist once told
me, that the most virtuous things to be reborn as in his order were:
1 Man
2 White elephant
3 Woman

XD

Though Chinese/Japanese Buddhism was hugely influenced by Nagarjuna,
they tend to downplay, or even be ignorant, of it. In some ways, those
branches go back to Buddha too. But they have a wordy disposition
where many of their masters write completely upfront that rebirth is
to be understood somewhat like a log consumed by flame still leaving
ashes – Dogen Zenji (Japanese guy) says burned logs don’t go back to
being logs again XD But I think even in China and Japan, the monks
don’t really rush to correct people in need of consoling fantasies of
literal rebirth.

Morality — Many Buddhists have some kind of morality. Compassion,
don’t hurt people, don’t cheat and lie, mmmkay. However, the Buddha
does not say that these are inherent truths about the universe. He is
actually pretty good on the is/ought on this point, even though Indian
thinkers don’t really think that kind of distinction makes sense. So
he says that it is only those *who wish to end the suffering that he
is speaking of* that should embark on that morality.
So the Buddha did postulate a certain morality. It was pretty fixed
and it was the intention that mattered (like Kant). But unlike Kant,
the Buddha also varied his own conduct depending on who heard his
sermons so maybe he broke his own precepts a bit XD
At any rate, the morality part has been overturned by many (but not
all) later Buddhists (even pre-Nagarjunists). There is a strong
tradition in Buddhism of “doing good brings no rewards”; “doing good
has no intrinsic value.” Some Buddhists (especially Indians) still
stick very closely to fixed precepts, but most don’t. Like the Dalai
Lama eating meat, or the zen masters beating up their own students.
Because fixed precepts don’t square so well with increased awareness
and the “mindfulness” you get in meditation, which is the *real* point
of Buddhism.

Meditation — This is where it’s at. Anyone can meditate and Buddhists
didn’t invent meditation. They were, however, the first ones (I think)
to say that everyone (who wished to cure the ills that the Buddha was
speaking of) should meditate and that meditation was kind of the point
of the whole thing. A lot of the other beliefs are just there to
support and mix with meditation. So, you don’t have to meditate, but
on the other hand, you can never understand the Buddhist teachings
fully if you don’t. There’s a realm of “intuitive knowledge,”
sometimes called the “dharma eye” because it’s unlike any other kind
of mental activity. It has nothing specific to do with Buddhism.
Hindus, Christians, Muslims etc. have such experiences too when they
meditate or engage in meditation-like behaviors. But Buddhism is
probably the only “religion” that takes the gist of these states and
builds the majority of its philosophy around that.

Enlightenment — The Buddha was enlightened in meditation, he saw the
truth about the universe with the “dharma eye” and saw that it was
seeing this truth (“we’re all oooooneeee”) could liberate people from
“suffering.” By suffering we mean something more like the fact that
phenomenal existence is imperfect and conditioned and therefore bound
to cause dissatisfaction because we could easily imagine the imperfect
to be perfect and we get attached to the idea of wanted the phenomenal
to be perfect. To experience enlightenment is not only to see how it’s
all oooooneeee, but also to see how that order is really perfect just
as it is (so there is a ceasing of wanting and attachments). That is
why I said that in Indian philosophy, Is/ought does not make much
sense – is/seeming would make better sense where seeming is phenomenal
existence and is is nirvana.

So nirvana is not some “place” like heaven and paradise; it’s just a
glimpse, perhaps only a second-long glimpse, of the “true state of the
universe,” glimpsed through the dharma eye. And since the mental
contents of this faculty are inexpressible, they cannot form
scientific claims that conflict with scientific experiments, etc. —
all they can do is argue with scientists about what is the “is” and
what is the “seeming” on the is/seeming divide. Is it the vision of
nirvana that’s the greater truth and phenomena that are lesser, or is
nirvana a hallucination with phenomena being the truth. Some Chinese
Buddhists (Hua Yen) actually made a whole branch in the middle ages,
arguing that each truth — the spiritual and the phenomenal — were
equally important.

Oh yeah, and some Buddhist branches, such as Soto Zen, don’t regard
enlightenment as important at all. Such experiences such create a
series of new attachments, because people dream themselves back to the
moment of their “vision” instead of focusing on their meditation.
That’s where the Zen teacher breaks out the stick and hits the pupil
for not paying attention to the now 😀

“No self” claim — It means several things: It means that you don’t
have an immortal soul, as in Hinduism or some Christian theology, for
example. It also means that no part of the brain is the “self,” like
Hume also says. If you _really_ examine your mental life without
preconceptions, you just experience a bundle of thoughts, feelings,
memories, fears, etc. – the “self” is a conventional belief that is
found not to have a direct, empirical basis in the faculties of
introspection.
“Life is suffering” claim — As said above. Suffering (dhukka) is more
like imagining/expecting the conditioned phenomena of existence to be
perfect when they can’t be: “Why am I not younger/smarter/richer/more
beautiful?” To stop this, the Buddha says that “the way” is to
meditate etc. as already mentioned. But it could be debated whether he
allows for there being other ways. Certainly, original Buddhism did
allow for people to keep their original religions and be Buddhist too.

Idealist/solipsism philosophy — Only Yogacara is solipsist/idealist.
The Buddha is actually something of an empiricist where it’s the
(inherently unstable) physical entities that come together to create
the faculties of consciousness. He is not a materialist/physicalist,
but acknowledges that there’d be no consciousness without the physical
prerequisites. And he *also* hinted that reality, as we perceive it,
is co-created by the mind (although understandably, he did not exactly
say how).

Rejection of the world — the
earliest Buddhism, they sometimes had morbid practices like meditating
in graveyards, not having children, and giving up all possessions.
This was to avoid attachments to imperfect phenomena and only meditate
and think of that. But Buddha himself rejected asceticism, as do many
modern Buddhists; they say that asceticism is just as much as bias as
indulgence, so let go of both!