It wasn't just Arsenal that failed to sign Cahill, Tottenham did so too. Note here that I refer to the club rather than the manager, that is because the manager actually has relatively little input in the negotiation process, as it is the chairman who conventionally leads transfer negotiations. Incidentally, it was reported that it was a direct intervention by Wenger that resurrected the seemingly dead Arteta deal. Bolton's valuation of Cahill now he's entered the final 12 months of his contract is clearly felt to be high of the mark, hence why two bigger clubs failed to secure the deal.
You'd be mad to argue you'd rather have the fool that is Barton at our club over Arteta. Arteta is a class above Barton, who I believe to be an average midfielder in the PL when it comes to ability and an absolute moron when it comes to personality. I fear having him at QPR will be a poisoned chalice for them.
I'm not really sure what more you can ask of Wenger; you practically want him to build an entire, world-class starting 11 in a single transfer window which is completely unrealistic.
Wenger is just one of a team of managing staff who all have their influence, and above him is the board and shareholders. Just because he's the face of Arsenal management with a seemingly all-encompassing title doesn't mean he is responsible for every wrong doing you perceive to have existed.
I'm guessing your voice wasn't one of the many fantastic fans who were cheering for Arsenal in Manchester last weekend. Their support made me proud to call myself a Gooner, other's unrelenting criticism saddens me.