Ray Allen vs other expiring contracts:
(1) If the team wants to win now, in the 2009/10 season, while also cutting their salary going forward ... then Ray Allen has a much larger trade value than other expiring contracts.
(2) If the team doesn't care about it's record in 2009/10, then Ray Allen has no more trade value than any other expiring contract.
Regarding #1: which team is a current title contender that also wants to dump good players to cut salary? I can't think of any, and even if there was one, I don't see any team which wants to win now parting with good players. A team that wants to win now will at most dump prospects and draft picks.
And even in that case, Shaq is still productive, Tmac is still productive.
(1) McGrady doesn't have trade value until he proves his health, and proves he can regain his previous level of performance. Shaq does though, absolutely, he's coming off a very good season. Jermaine O'Neal is a guy who has some trade value as a player, but a lot less than Ray would have.
(2) It's not solely contenders. It can be playoff teams in general, and also a team like NY who is trying to win now without effecting their 2010 money. Milwaukee is another type of team, a team who may want to cut their long term money (say Michael Redd), but are trying to be as competitive as possible at the same time.
That's four types of teams -- contenders, some playoff teams, 2010 orientated teams, and teams looking to cut costs while remaining competitive -- not just contenders.
(3) It is a limited market (both because of the teams, and the level of talent needed in return to make it worthwhile) and there may be nothing worthwhile there. There's no guarantee that there will be quality offers there. After Danny Ainge explores the trade market, there's a high likelihood that there's nothing there.
(4) Most of the Ray Allen trade ideas will be possibilities rather than certainties, teams which may consider it -- like the Josh Howard and Dallas or the Martin and Sacramento idea above -- I think there will be very few ones that you look at and say that will definitely be on the table.