Count the pieces that have left the back rank. That number, for both sides, decides more beginner games than any opening name.
Development is the work of the first ten or twelve moves: knights and bishops out to squares where they attack something, rooks connected on the back rank, king castled away from the centre. A piece sitting on its starting square is a piece your opponent does not have to think about.
Five moves, five useful jobs
8 
7 
2 
a 
b 
c 
d 
e f g h 1 Here is what that leaves on the board.
7 2 a b c d f g Nothing clever has happened. Every move put a new piece on a square where it does something: the knight on f3 covers e5 and d4, the bishop on c4 stares at f7, castling puts the king behind three unmoved pawns and brings the rook to f1. That is what a good opening looks like from the inside. Dull, and very hard to beat.
What moving twice costs you
Every move you spend on a piece that is already out is a move your opponent spends on a piece that is not.
Say you play 3.Bc4 and he attacks it with a pawn, so you retreat to b3. Two moves for one bishop, while he has brought out a knight and a bishop. Do that three times and you are four pieces to one behind on the queenside, which is how people lose in twenty moves without ever hanging anything.
The same goes for early queen sorties. The queen comes out to h5, gets chased by Nf6 and g6, and goes home. She developed nothing; the moves she cost you developed his whole kingside.
The check before move twelve
Ask three questions before you start a plan. Are both knights out? Are both bishops out? Have I castled? If the answer to any of them is no, and there is no tactic on the board, your move is probably the one that fixes it.
Being ahead in development is worth something concrete: it is where the initiative comes from, and it is the reason gambits work at all. It also expires. Trade the pieces and the lead is gone, so a development lead has to be used while it lasts, usually with a threat on every move.
More on how the phases fit together is on the phases of a chess game, and if you want to see development turn into an attack, the opening puzzles are full of positions where one side simply got there first.