The discovery of pulsars in 1968 by Hewish, Bell et al. was unexpected and had an immediate impact. It was followed by a quick stream of papers : after a brief “little green men” interlude 1, the focus quickly moved to the current spinning neutron star explanation. Why did it get such a quick response ? Probably because the basic theoretical framework was already in place, from Baade & Zwicky (1934) and later theoretical developments: people quickly made the connection.
Another example is the discovery of the high fractions of dark matter in dwarf spheroidal (dSph) galaxies, starting with the work of Faber & Lin (1983) and Aaronson (1983). The arguments were not strong at the time and the discovery came as a surprise, but there was only limited scepticism. Dark matter in dSph galaxies remains a very active field today. Current estimates of their mass-to-light ratios exceed 1000 solar units (e.g. Wilkinson et al. 2006) for some of the faintest dSph galaxies. Why did this discovery get such a quick response ? Probably because the basic observational infrastructure of dark matter in brighter galaxies was already there, plus the theoretical ideas on the role of dark matter in galaxy formation (e.g. White & Rees 1978).
1 My recollection from that time is that the discoverers briefly considered the possibility that they had detected a signal from an extraterrestrial civilization, but soon discarded this possibility as further pulsars were discovered. Back.