How is the most mainstream metaphysical view different from Kantian metaphysics? Kantian metaphysics used to be the dominant metaphysics during Kant's time, but a lot happened in metaphysics, science and mathematics, so that many of the Kantian metaphysics also had to be tweaked into something that I consider to be the mainstream metaphysical view, because I haven't spent nearly enough time to study metaphysics, I don't know what that mainstream metaphysics view or theory is different from Kant's metaphysics.
What is the "mainstream metaphysical view"?
According to the only more or less reliable source I am aware of, which is the PhilPapers survey of philosophy professionals, the mainstream view is
Accept or lean toward: non-skeptical realism 760 / 931 (81.6%)
This assumes that you take metaphysics to mean ontology. If you follow the Kantian trajectory, metaphysics cannot be thought independent of epistemology and has to be sceptical/critical.
Funny enough, if you look at the other questions, while there is a clear tendency towards externalist, realist physicalism - which is not Kantian - the results suggest at least some degree of inconsistency between ontological and epistemological beliefs even among philosophy professionals.
How does this relate to Kantian metaphysics?
As already said, this hinges a lot at how epistemological and ontological beliefs are linked. I would guess from the above survey that most philosophers hold some kind of (modified) representationalism, ie. believe that the contents of our consciousness are representations mediated by concepts and knowledge, not some magical direct insight into the fabric of reality itself. This is a Kantian core insight.
The major differences are the following two:
1. The a priori which structures our perception and possible knowledge is not universal, eternal and immutable
Modifying Hegel, philosophers tried to account for cultural plasticity (and contextualisation) of perception itself. This means that a core belief and foundational stone of Kantian epistemology has been shattered and that in a sense, the main drive of his, providing a certain and irrefutable basis for philosophical knowledge against skepticism, has to be considered a failed project.
2. Scientific inquiry eventually transcends the difference between phenomenon and noumenon
Kant held that there is a categorical difference between our representations of reality (structured by the synthetic a priori) and that which is represented by them. These days - and that is why people can hold that physical, external things are (more or less exactly) as we think them to be - the mainstream view is that via scientific inquiry our representations come ever closer to reality itself.1 Since our scientific insight modifies how we perceive the world, the structure and content underlying our perception and knowledge is increasingly congruent with the structure and content of the world itself. Kant fiercely rejected that such a claim would be justifiable and thus said it was mere speculation.
Since we increasingly justify pragmatically - nature behaves exactly like we expect it to, thus it does not make sense to uphold a "transcendent" realm different from it underneath - this position was given up.
To be fair, the noumenal realm was mainly a construct which allowed Kant to have some space left for a God and metaphysical free will. Since modern metaphysics do not necessarily care about these two, the importance of that difference has lessened accordingly.
1 Strictly speaking, this formulation is a bit misleading due to another difference between contemporary philosophy and Kant: Kant held that the term "real" should only be applied to phenomena plus the Moral Law, and that it is speculative philosophy (or no philosophy at all) to speak about how the noumenal or an alleged "reality itself" - in the sense of "that which is the cause of phenomena or to which they correspond" - is like at all (see eg. Opus Posthumum, Ak 21:75)