A classical analysis might explain observations 1 and 5 above. But nothing in this analysis suggests that there should be a threshold frequency, as Lenard found. If a weak intensity at a frequency just slightly above can generate a current, why can’t a strong intensity at a frequency just slightly below do so?
What about Lenard’s observation that the current starts instantly? If the photoelectrons are due to thermal emission, it should take some time for the light to raise the electron temperature sufficiently high for some to escape. The experimental evidence was in sharp disagreement. And more intense light would be expected to heat the electrons to a higher temperature. Doing so should increase the maximum kinetic energy of the photoelectrons and thus should increase the stopping potential . But the stopping potential is the same for strong light as it is for weak light.
Although the mere presence of photoelectrons did not seem surprising, classical physics was unable to explain the observed behavior of the photoelectrons. The threshold frequency and the instant current seemed particularly anomalous.