I'm going to play devil's advocate here, wether you feel it is a house of cards or mirrors is up to you ;)
"What surprises me is that the attackers messed up so badly."
1) That is an assumption we are all making, and it is only based on our opinions (if none of them are ever found etc then no they did not mess up).
"Sure, a decade or two ago this would have been a perfect, undetectable kill."
2) Yes and it is right out of the (supposed) Mossad "play book" as written 30 odd years ago by a (supposed) Mossad defector who tried to crack down on Mossad coruption.
"Today it is not and it is not so surprising that they were caught"
3) Actually they have not been caught a set of pictures and names that did not belong to them have been published by the Dubai Police.
"given the high-profile target."
4) Let us assume for the moment they knew due to the nature of the target that what has happened was very likley to happen publicaly or otherwise.
"It would possibly have still gone undetected with a low-profile target. Scary thought."
Yes and people have reason to belive that the previous UK Prime Minister R.Hon Tony Blair PM, MP had Dr David Kelly "suicided" for the same reason "scooter" libby outed a CIA agent. We certainly know that the head of the UK Civil Service got politicaly involved (a down right no no) and had Dr Kelly threatand just before he gave evidence to a House of Commons Commity. So politicians are quite happy to play "wet games" of one form or another.
"Anyways, have these killers gotten rusty or used procedures from a long time ago?"
5) This is the sixty four thousand dollar question.
If you look back to (2) you might think "hang on a moment"...
Let us assume that they have not gone rusty but deliberatly used old Mossad tactics as described in the book...
It gives rise to an interesting set of posabilities (which I'll go into later).
"Did they not scout the location adequately?"
6) It has been said that the target changed hotel unpredictably (but more on that later).
"Or were they so sure of sucess that they got sloppy"
7) You need to re think that a little bit as,
'got sloppy' or were 'deliberatly sloppy'
It ties back to point (5).
"and failed in what must have been one of the two primary mission objectives?"
8) again you are making what may be an incorrect assumption.
They may well have decided as part of the mission plan to deliberatly play it this way.
"At least the little wave one of the killers gave to a surveilance camera seems to suggest this."
9) May well have been the equivalant of an "up yours" or 1
/ 2 finger salute.
By somebody who knew perfectly well that even a full on face shot to camera was not going to do anybody any good because their planning had mitigated this (more on this later).
"Anywasy, this is a massive screw-up."
10) That depends on your assumptions about what the game plan was.
"Killing somebody is not hard. Any caveman can do it. Getting away with it is the tricky part."
Easiest way to do this is "drunk driving" in a "stolen car".
11) The Police tend to belive what their perception of the facts tell them.
"I think these people have just failed "Assassination 101" by reason of a beginners mistake."
12) You may be making a police forensics 101 mistake.
Back to the Devil's advocate argument,
'The plan went without a hitch and actually worked better than expected. The target is dead and it appears to have been done to an old well published Mossad play book, thus Israel are getting the blaim as expected.'
A) The plan assumed that the death of a very important target would get more than Police forensics 101.
B) The plan was then designed around this (A) and was staged appropriatly to look like Mossad.
Appart from they "failed field craft 101" this gives rise to two basic possabilities (B.1, B.2).
So the three basic possabilities are,
B.1) It was a group trying to make it look like Mossad.
B.2) It was Mossad trying to make it look like Mossad.
B.3) Who ever they are they failed field craft 101, for one reason or another.
Thus bluff, double bluff or circumstances / ineptness (you could argue tripple bluff etc but that takes you back to B.1 or B.2 anyway).
If any of the people get positivly identified over and above matching the photos and convicted then yes it's B.3.
C) But B.3 breakes down into another issue.
It may not have been possible to get the target without being identified.
Thus for some reason the cost of 30 field agents was considered justifiable.
Thus you have several possabilities arising,
C.1) The target had to be stopped to prevent a more serious issue.
C.2) The the death of the target was not the plan.
C.3) The death of the target was for Political not tactical reasons.
I'm going to commit the grave sin of following "Godwin's law" for a moment as a historical point.
We know that there where several plans to take out Hitler during WWII but although some where reasonably viable (on paper) they did not go ahead.
One of the reasons stated later was that he was more use alive than dead. This had some creadence due to the German's own military trying to kill Hitler.
So lets flip that one on it's head.
D) Let us assume that the target is more of a capability threat than the others in his organisation or even other similar organisations, or was trying to broker some kind of deal.
Thus the target may have been planning to,
D.1) Remove those less competant than himself.
D.2) Form an aliance with another organisation.
D.3) Broker a deal with a hostile forign power.
D.4) Broker a "peace deal" which would not be in the interests of those whos organisation/nation the team came from.
I could go further but I'll stop to let people have a think and give their two cents worth.