What justification can you give for the fact that "most ODEs do not have an explicit solution"? What justification can you give for the fact that "most ODEs do not have an explicit solution"? 
 A: If the ODE is linear --and the notion of «explicit» refers to Liouvillian solutions (towers of iterated quadrature and exponential of meromorphic functions)-- then its differential Galois group (Picard-Vessiot theory) must be a solvable algebraic subgroup of $GL_n (\mathbb C)$. Such subgroups are rare: they define a proper algebraic subvariety. The defining equations correspond to trivial commutations relations, e.g $[G,G]=0$ in the Abelian case, encoding the tower of normal subgroups intervening in the definition of solvable group.
In the non-linear context the intuition is the same: nice «transverse structures» are rare.
Edit: for a statement regarding the non-linear case, see my paper http://fr.arxiv.org/abs/1308.6371v2, Corollary C.
A: On the non-linear side, there is a theorem of Hudai-Verenov MR0147699, which says that for
a generic (open dense set) equation $y'=R(x,y)$ the graph of a generic solution is dense.
See also Ilyashenko, MR0247176. This implies that this equations do not have first integrals, so they are not "solvable" in the sense that we teach in the elementary courses.
This is an example of a general theorem on the subject. But of course it is well known from the last 300 years of experience with differential equations. For example, all integrable cases of the rotating top in uniform gravity field are explicitly known, and they are exceptional. There are many other examples like this.
