A few brief comments, just using my head:
(i) DH: traditionally it was more "down the hill", i.e. schuss, now the new tracks are more tecnical with more curves (turnier, as sjj writes) to suit better-turning new DH skis.
The last "old school" downhillers like Russi, Klammer, Grissmann, the Crazy Canucks, Muller, Alphand and others were, quit or are not far from it (Trinkl, Ghedina, Fritz Strobl). The new type of the DH racer prevailed and in about 1997 the Austrian guys from "Weltcup III"-group (Maier, Eberharter, Schifferer, Pepi Strobl, Knauss) started to dominate both the speed disciplines as the world cup itself.
The speeds are somewhat up although oficially everybody says they should be reduced, e.g. due to more difficult and less highway-like trails.
The skis: not much different length- and radiuswise (45-55 m) but softer.
(ii) SG: since 1984 on the WC
Formerly slower, more "slalom"-like (cf. the name "supergiant slalom"), now often very fast with speeds over 60 mph
In the beginnings, some excellent technical skiers were outstanding SG racers, e.g. Deborah Compagnoni who won her first Olympic gold in SG. Very few know that Alberto Tomba, known for his concentration on technical disciplines only, was ranked 5th in SG in 1988!
I had also mentioned the SGs Ingemar Stenmark took with the best 5th place in Garmisch 1984.
It´s debatable whether a race with such speeds should be held without the prior training runs.
The skis: somewhat shorter than they used to be (cf. the minimum length required by FIS) but Schifferer still on 210 cm (told me on the lift personally, I guessed around 209 cm), radius IMO around 35 m, softer
(iiii) GS: very fast in high-level competition, the trend toward "less carving" has been discussed in an other thread
used to be less fast - I just came across some FIS complaints from about 1980 that GS is no speed event anymore but closer to SL - which would indicate some changes throughout history
(iv) SL: seems to be faster owing to less friction and less loss of speed in the turns, similar trend as in GS, the maximum gate distance is used more often
is simpler and less complicated than it used to be (no complicated "slalom figures" but more simpler gates/combinations which is the only possibility in the speeds attained).
Both GS and SL skis are being discussed so often that I find any special comments not necessary.
It could be a big topic. Mine are just modest 2 cents offhand.