Looks like this tie has really caught the board's imagination
My thoughts:
Clearly it's a very interesting matchup. I feel Barça are narrow favourites, but it's pretty close to 50-50 for me. In the CL it all depends on how the individuals perform on a given night, and any mistakes/off-colour performances carry the maximum penalty of a knockout. We've seen that the Arsenal players tend to do stupid things quite often, but you can't say they won't pull it together for 2 legs.
We all know that both teams like to keep possession and play a lot of passes before attempting to shoot. The shape of both sides is also identical, esp. this season with Wenger changing to a 4-3-3 (IIRC he commented on its usefulness for Arsenal after seeing the CL final last year). So IMO the advantage is created by the team that can:
1. Keep the ball for longer
2. Win the ball back as fast as possible
In both cases I give Barça the advantage, because our players' circulation of the ball is much faster than Arsenal's, and the pressing when the other team has the ball is more consistent than Arsenal's.
However it's also key that the team doesn't make the tactical mistakes that they've made in some games this season. For example the 1st half against Valencia recently: the defensive line was not high enough, and the passing was too direct. If the defensive line pushes up close to midfield, it reduces the amount of space that Arsenal have to play in.
And while Arsenal get lots of teams trying to stop them from playing with physical tactics, I don't think they get too many teams that stop them from playing by eliminating their time and space on the ball. So a repeat of the suffocation tactics from the 2nd leg against Stuttgart will be important if Barça want to have a more comfortable match.
It's also a waste of time to compare players in like-for-like positions. What you need to do is compare the matchups in different areas of the pitch.
Clichy has the pace to handle Messi, but I don't think he can take care of both Messi and Alves. Presumably Song or Denilson will have to help him out...if it's the former, that takes away a key player from the centre of Arsenal's midfield. If it's the latter, then it's no problem for Messi and Alves to still create danger.
Then on the left you have Sagna against either Henry or Pedro. I really hope Henry doesn't play in the 1st leg at the Emirates. Not that I doubt his professionalism, but like Beckham at Old Trafford, at the back of his mind he'll want to try and be "nice" to the Arsenal fans, not do anything that might annoy them. Plus he doesn't really have the pace that would trouble Sagna. Same goes for Pedro, he's good at scoring goals, but lacks the ability on the ball to trouble someone fast and experienced like Sagna. I'd play Iniesta here...he's not particularly fast, but if you get the ball off him fairly, give yourself a pat on the back.
Song is a very good player, I think he'll be able to deal with our central attacks and give good cover for Campbell. Because if you get Messi or Iniesta running directly at Campbell, that will create problems for Arsenal. The other thing to watch out for is passes behind the defense that turn Campbell/Vermaelen. But for this to happen, Henry needs to play in the middle, because Ibrahomovic doesn't seem to make those runs.
Defensively for Barça, the only major worry is Arshavin's shooting and dribbling ability from the left, esp. against Alves who will sometimes be backtracking like a madman.
Our back 4 gives up a few chances each game, and if the other team is efficient enough, they can take advantage. Pique will handle Bendtner, and if Abidal is back, his pace means there won't be problems from whoever's on the right.
Setpieces are usually a problem for our defenders but apart from Campbell and Vermaelen there's no real threat from Arsenal in this regard. Indeed, Puyol/Pique/Ibrahimovic/Keita will probably fancy themselves against the Arsenal defense, which we know is quite disorganised on corners.
Cesc will be influential regardless of whether Toure or Busquets play. I'm not getting into the pointless Song vs. Busquets debate, but I'll say that the criticism of Busquets is fast turning him into an underrated player from the slightly overrated player that he was until now. De Guzman is right though when he says that the Arsenal midfielders, esp. the likes of Nasri and Rosicky, should run directly at him and try to draw dangerous fouls.
In fact, now that I think of it, I'd like to see Keita in the midfield. His energy and ability to go box to box can really unbalance Arsenal. I know most Barça fans on the board won't agree with me, but this is the starting lineup I want to see for the first leg:
Valdes
Alves-Puyol-Pique-Abidal
Toure-Keita
Xavi
Messi-Ibrahimovic-Iniesta
Pep may try to squeeze Marquez in there, I really hope he doesn't
Edit: Forgot to mention...Barça seem to trade victories with all the English clubs in recent years. Happened with Chelsea and Man. Utd., so if this "revenge theory" is an omen (may have some truth to it from the aspect of a greater hunger to win), then Arsenal will go through