Rewatching the first leg of the 08/09 CL semifinal between Barça and Chelsea. Barça dominant in the first half outside of one misplaced back pass from Rafa Márquez, but what's really standing out to me is how open Chelsea were in midfield.
They were primarily in a 4-1-3-1 shape on the defensive end, with that 3 in midfield staying very flat most of the time. It was just so easy for Barça to progress the ball, and they had so much space in front of the Chelsea defensive line. Several examples of this below

Leaving your centre back on that much of an island with Henry? Then after he makes the run to the right and pulls the defender away, Iniesta moves into the huge gap, but there wasn't enough weight on Márquez's ball.
Space for Toure to drive into or lay the ball to Xavi, who can then easily cut inside and look for Eto'o or Iniesta.
There were almost always gaps in that Chelsea 3 for a straight ball to the forward line (Henry in this case). Then because the midfield is so far separated from the back line, there's nobody there to double team or put real suffocating pressure on Henry.
Giving Messi that kind of room between the lines? Toure actually ended up just playing square to Iniesta here lol
Messi allowed to check in, receive the ball, turn, split two more defenders to find Henry, get the return pass, and have a shot.
Wide open in the middle. Messi just jogs towards the center and finds himself receiving the ball with acres of space to drive into.
Another thing: When Xavi or Iniesta danced around one of those 3 in the Chelsea midfield, which they were obviously capable of, things became wide open.
Iniesta driving forward once again. He sets up Eto'o in a very dangerous position to go to goal, but it's ruled offsides. Close one for sure
This is all just from one half. I’m not too sure what Chelsea were trying to do here, but it really seems like one of the worst ways you could play against this Barça side, right?
Maybe they thought the line of 3 could match up with Iniesta, Toure, and Xavi and shut them down, with the 1 deeper midfielder cleaning up behind them.
Maybe they thought having that flat 3 would form some kind of a wall in midfield that closed down progressive passing lanes.
But how it turned out was such an open, easily exploitable setup. The line of 3 were far too spread out, leaving massive gaps between themselves horizontally the back line vertically. Plus they didn’t even press with much intensity at all
This left the 1 deeper mid behind them to cover an ocean, and he and the centre backs often got dragged out anyways.
Whoops. Meant to say their defensive shape primarily looked like a 5-1-3-1, not a 4-1-3-1. Sometimes also looked like a 4-1-4-1
Also, for a team of intelligent footballers, Barça's shot selection was poor. Quite a few long range efforts when there were better options on. Seemed like it could've been due to nerves honestly