For instance, deficiencies of C3, factor H or Factor I result in recurrent pyogenic (bacterial) infections, which is consistent with C3's role in opsonisation.
Furthermore, you'd assume that deficiencies of the 'final pathway' components C5 to C8 would lead to less lysis of microrganisms, and you'd be correct. A weird thing though: it seems as though your susceptibility to Neiserria (meningitidis or gonorrhoea) is particularly increased. Another weird thing: a the lack of C9 seems not to be clinically relevant.
Lastly, defects of the classical pathway components (C1, C2, C4) result in a predisposition to develop 'immune complex' disorders like SLE. This is consistent with the classical pathway's role in clearing immune complexes.
There! See, not so bad? That's most of it, anyway...