I tried to classify all openings in three categories:
- Sharp and theory-heavy (the openings where one single mistake can be fatal)
- Intermediate and balanced
- Slow, quiet and theory-light (the openings that anyone can play without needing to study them and memorize tons of moves)

I'm only talking of statistical averages here. Like, for example, I put the Sicilian in the category "sharp and theory-heavy", which means that on average the Sicilian generally tends to lead to sharp positions, most variations of the Sicilian are very theory-heavy. But there are a few exceptions, such as the Taimanov and the Kan (Paulsen) for Black, the 6.Be2 Open Sicilian and the 3.Bb5 Sicilian and the Closed Sicilian for White.

Almost all gambits are very sharp and theory-heavy.

For White:
- Systems tend to be slow, quiet and theory-light.
- 1.e4 tends to be a bit more sharp and theory-heavy than 1.d4, 1.Nf3 and 1.c4.
- Three Pawns Attack (f4, e4, d4) and Four Pawns Attack (f4, e4, d4, c4) are very sharp and theory-heavy.
- f3 (e.g. KID Saemisch) tends to be sharp and theory-heavy.
- An early Nf3 while the f-Pawn is still on f2 tends to be a bit more slow, quiet and theory-light than average.
- g3 (Fianchetto variation) is slow, quiet and theory-light.

White plays 1.e4
--- Sharp and theory-heavy ---
Open Sicilian all variations except 6.Be2
French 3.Nc3
Caro-Kann Advance 4.Nc3 e6 5.g4
--- Intermediate and balanced ---
Caro-Kann Panov-Botvinnik Attack
Scotch
Ruy Lopez
Italian
--- Slow, quiet and theory-light ---
Four Knights
6.Be2 Open Sicilian
3.Bb5 Sicilian
Closed Sicilian
Caro-Kann Exchange 4.Bd3
Caro-Kann Advance Short (4.Nf3 e6 5.Be2)
French Tarrasch (3.Nd2)
French Exchange

Black against 1.e4
--- Sharp and theory-heavy ---
Sicilian
Pirc-Modern
Alekhine
Philidor
--- Intermediate and balanced ---
French
1...e5
Sicilian Taimanov
Sicilian Kan (Paulsen)
--- Slow, quiet and theory-light ---
Caro-Kann
Scandinavian

Black against 1.d4, 1.Nf3, 1.c4
--- Sharp and theory-heavy ---
Gruenfeld
Modern Benoni
KID
Semi-Slav
--- Intermediate and balanced ---
Nimzo
QID
King's English (Reverse Sicilian)
QGA
Slav ...Bf5
Slav Chebanenko/Chameleon (4...a6)
--- Slow, quiet and theory-light ---
QGD
Symmetrical English

I not only classified the openings in three categories, but I also classified them in descending order of sharpness and theory-heaviness.

Tell me what you think. Do you agree? Which openings do you think I misclassified? I am particularly uncertain about the openings that I put in the "Intermediate and balanced" category. You can post your own list.