Okay, here are my thoughts, they are quote random but I will add if I remember more:
Centurion Enroute remains one of the best benefits
Most executive are generally very nice and have manners. However, the quality of the service is often inconsistent, lack of follow through, and slow. Amex ancillary services such as FX and third parties such as ACE doesn't provide a good enough customer service.
Travel - I have most issues with them - the last good trip I had with them was in 2007. Last year I wanted a holiday design for me and gave them some general parameters: I need to be at the conference at a specific destination at a certain date; I need to be in a certain country during the trip but it does not matter when, and listed out the sort of things I like to do. It is too hard for them, everyone wants to know my routing details from me, and keep telling me that it is too difficult for them if I don't have routing details. On my return, they advertised on the Cent magazine about "Crafted Journeys"... I complained to Travel's team leader, my RM, my business account RM, and the Asian Pacific premium card head. Some replied, and some promised a contact in the future and none has come through. Travel pearls such as how to route to save, or something as basic as checking price difference between two alternative ticketing options would have been useful.
Concierge - only if you find the right person which I do have someone who has been really good especially on restaurant recommendation around the world especially Japan (getting the reservation is another story). I have a concierge I always go through if possible and she has been able to achieve some impossible for me. However, recently these are things that I have been told that was too hard - such as sending flowers to my wife on a Sunday, or finding a track day in Dubai/Abu Dhabi.
I feel that it would be much better to have an assigned travel agent and a concierge could be useful.
I am actually happy to pay the annual fee, but it is high compare to the benefits. Australian Cent remains one with the least benefits compare to the same card around the world - there are really not many money can't buy benefits. Plat and Cent benefits are too close.
Recognition over Platinum are usually poor. Most hotels will just pull out the Plat benefits and make no distinction between the two.
Inconsistent recognition for status - for example IHG has been excellent and consistent, but Hilton Diamond is a bit useless as they can see that you are not a "real" Diamond, and always scheme on the benefits. However, my tendency to stay at FS hotels makes most of the status irrelevant.
ACE claiming is a PITA - taken weeks to claim my hire car excess waiver and they have knocked back 2 out of 3 of genuine claims on technicalities on the policies. The amount of time I waste it would be easier if I pay for excess waiver on my next car hire.
CX Diamond - the fact that you can get a seat even when there isn't a reward seat available makes that the best benefit the Cent ever had. It is a way for me to use up my points. EK Gold - does not equate to CX Diamond at all.
For me lounge access at airport is a bit useless as I don't fly economy. However, if there are F lounge access then it would be useful as often you are on J not by choice but because the route doesn't offer F.
I like to see networking functions, or unique experience that money can't buy. Access hard to get table in restaurants would be nice with minimal notice would be nice, the Cent table are often booked months in advance on some of the very popular restaurants.
Langtons has been good so far.
Card replacement is extremely quick, and RM has always been very nice. However, there are only limited things they can do if you have a complaint.
I like the tripadviser link. However, it would be nicer if you can fine tune to Cent holders or Cent/Plat holders recommended.
Overall, I would prefer a much more personalised service.