Thanks much Eric. I believe WRI's position will be this is intended behavior since as you point out the documentation has, for a long time now, read that the first argument is treated as a function. I too wonder why it used to work and now it does not. It appears not to be a subtle task to move all terms of an equation expression to one side of == and then get rid of the == resulting in an expression which can be treated as a function.

@Eric, I believe you may have misunderstood what I try to do. I am not trying to plot the solution. I try to plot the equation. Are you amazed that

Plot[Evaluate[2 x^2 + 3 x == 20], {x, -5, 5}]

ever worked? The above yields empty plot in Mathematica 10, but not in 8.0.4 and 9.0.1.

When I try to plot the differential equation with the solution replacement, what I am trying to do is see how well the equation was satisfied with the solution found.

You should then be equally amazed that

Plot[Evaluate[eqn[[1]] - eqn[[2]] /. sol], {t, 0, 5.}, PlotRange -> All]

does work in Mathematica 10, and is equivalent to plotting the equation with solution replacement, although somewhat more burdensome when many equations are present.