Thierry Henry points to his age, the uncertainty of Arsene Wenger continuing on as manager, and the departure of David Dein as reasons in his leaving.
They have their merits but Arseblog rightly points out that the David Dein reason is a bit dodgy. However, in retrospect, it does appear that Dein's departure shook him up more than anything else. Henry's personal connection with Dein's family cast a cloud on the whole transfer saga. At the end of the day, we might convince ourselves that Henry is a professional and a friendship should be viewed as just that. But these connections can be complex and potentially influential as we saw how the friendship between Abramovich and Sheva complicated matters at Stamford Bridge. Dein's leaving may have left its mark on someone as moody and enigmatic as Henry.
A lot of this is also rationalization to make Henry's departure palatable to Gunner fans. At the end of the day, he wanted to play with the big names, Samuel Eto'o, Ronaldinho, Leo Messi, Deco, and Zambrotta. A bit self serving but one can't really fault him on that. He has proven himself above and beyond the challenges of the Premiership but now wants to go to the next level. Even Didier Drogba has mentioned leaving Chelsea to play for the Serie or La Liga in the future as a serious possibility.
This leads us to the question, what will be Arsenal's future? A player of the caliber of Henry leaves after having scored 226 goals in eight seasons, leaving a club bereft of yet another proven goalscorer. The first order of the day would be to replace him with yet another player of similar caliber. At this point names like Anelka, Fernando Torres, Samuel Eto'o are purely in the realm of conjecture and Arsenal needs to sit down and seriously begin negotiating because they are now competing with a lot more clubs with money. Newcastle and Tottenham have already signaled their intentions that they are ready to take it to the next level with their recent managerial and player acquisitions. Chelsea, Man Utd, and Liverpool have not missed a step yet.
In all of this, one must not forget that as much as we need a star quality player, we also need a captain. Players like John Terry, Steven Gerrard, and Gary Neville bring this flinty quality to their clubs which help rally players. As much as skillful feet, is the need for heart and soul. And we have lacked that from Henry. In a youthful club like Arsenal, this might mean someone like Gilberto but even Cesc is showing his chops as a leader. In all of this, we must not forget the foundation that brought Arsenal its success, Wenger's commitment to developing talent long term, the unerring eye that sees potential in a relatively untested player, and the type of match play that has made Arsenal unique and a joy to watch.
For Thierry Henry, thanks for all the beautiful memories and the wonderful goals that graced Highbury. We move on (hard swallow).