Anything by John Milnor fits the bill. In particular, "Topology from the differential viewpoint" made me feel that I understand what differential topology is about, and the "h-cobordism theorem" made me feel that it's beautiful. Many other books and papers by him are wonderful; the first that come to mind are "Characteristic Classes", "Morse Theory", lots of things in Volume 3 of his collected papers.
Many topics in linear algebra suffer from the issue in the
question. For example:
In linear algebra, one often sees the determinant of a
matrix defined by some ungodly formula, often even with
special diagrams and mnemonics given for how to compute it
in the 3x3 case, say.
det(A) = some horrible mess of a formula
Even relatively sophisticated people will insist that
det(A) is the sum over permutations, etc. with a sign for
the parity, etc. Students trapped in this way of thinking
do not understand the determinant.
The right definition is that det(A) is the volume of the
image of the unit cube after applying the transformation
determined by A. From this alone, everything follows. One
sees immediately the importance of det(A)=0, the reason why
elementary operations have the corresponding determinant,
why diagonal and triangular matrices have their
determinants.
Even matrix multiplication, if defined by the usual
formula, seems arbitrary and even crazy, without some
background understanding of why the definition is that way.
The larger point here is that although the question asked about having a single wrong definition, really the problem is that a limiting perspective can infect one's entire approach to a subject. Theorems,
questions, exercises, examples as well as definitions can be coming
from an incorrect view of a subject!
Too often, (undergraduate) linear algebra is taught as a
subject about static objects---matrices sitting there,
having complicated formulas associated with them and
complex procedures carried out with the, often for no
immediately discernible reason. From this perspective, many
matrix rules seem completely arbitrary.
The right way to teach and to understand linear algebra is as a fully dynamic
subject. The purpose is to understand transformations of
space. It is exciting! We want to stretch space, skew it,
reflect it, rotate it around. How can we represent these
transformations? If they are linear, then we are led to
consider the action on unit basis vectors, so we are led
naturally to matrices. Multiplying matrices should mean
composing the transformations, and from this one derives
the multiplication rules. All the usual topics in
elementary linear algebra have deep connection with
essentially geometric concepts connected with the
corresponding transformations.
Best Answer
The world's output of scientific papers increased exponentially from 1700 to 1950.
One online source is this article (which is concerned with what has happened since then). The author displays a graph (whose source is a 1961 book entitled "Science since Babylon" by Derek da Solla Price) showing exponential increase in the cumulative number of scientific journals founded; an increase by a factor of 10 every 50 years or so, with around 10 journals recorded in 1750.
Perhaps someone can locate similar statistics specific to mathematics, but it's reasonable to expect the same pattern. If so, it is a long time since any individual could follow the primary mathematical literature in anything close to its entirety.
But then, gobbling papers is not how leading mathematicians (or scientists) actually operate.
By making judicious choices of what to pursue when, and with sufficient brilliance and vision, it is possible even today to make decisive contributions to many fields. Serre has done so in, and between, algebraic topology, complex analytic geometry, algebraic geometry, commutative algebra and group theory, and continues to do so in algebraic number theory/representation theory/modular forms.