Merteszacker seems less solid than in Bremen because the people before him don't defend enough. Alex Song is perhaps my favourite Arsenal player, in the matches i've seen him (live) this year (against Blackburn and Chelsea) he had a couple of fantastic moments with beautifull passes that lead to goals (cfr. the Santos goal against Chelsea), but defensively he has too much weaknesses to be a DMF...I've already said it countless times, Arsenal's top priority is a good, commanding, cynical DMF. A player who is a leader on the pitch and who wants to play ugly amid all this artistic football...every great team has such a player...The first big Arsenal team under Wenger had Viera, Man Uts has had Keane, Ajax had Neeskens, Barcelona has Busquets...some of these players are very good players, but that is of lesser importance (of course it's a bonus). The big Dutch team in 1974 had Neeskens, Van Hanegem, Rijsbergen, Haan...plenty of talented but also very cynical players...
IMO that is the only thing Arsenal misses to win silverware...