  We give a brief review of violations of the fluctuation-dissipation theorem
(FDT) in out-of-equilibrium systems; in mean field scenarios the corresponding
fluctuation-dissipation (FD) plots can, in the limit of long times, be used to
define a {\em effective temperature} $\teff$ that shares many properties of the
true thermodynamic temperature $T$. We discuss carefully how correlation and
response functions need to be represented to obtain meaningful limiting FD
plots in {\em non-mean field} systems. A minimum requirement on the resulting
effective temperatures is that they should be independent of the observable
whose correlator and response are being considered; we show for two simple
models with glassy dynamics (Bouchaud's trap model and the Glauber-Ising chain
at zero temperature) that this is generically not the case. Consequences for
the wider applicability of effective temperatures derived from FD relations are
discussed; one intriguing possibility is that at least the limit of the FDT
violation factor for well separated times may generically be
observable-independent and so could yield a meaningful $\teff$.
