I'm now thinking that what the GMAT really tests is different modes of thinking. GIN's posts provide the clearest hint:
* Reading Comprehension: understanding and digesting information that is read
* Sentence Correction:
- not being phased by question
- thinking through all rules/concepts and determining which apply

For Data Sufficiency:
For each of the two statements, and then them combined:
- thinking of the different values that can be punched in to test different scenarios. e.g. with a formula question, going through negative and positive values combinations for different variables
- often one statement has more information than it seems
- rephrasing questions to make the formula/question easier understood
- determining different values that can be ascertained from information

The test is also a test of stamina and concentration over a long period of time.

http://gmatclub.blogs.com/blog/2004/07/gmat_prep_tips.html

Study tips that every single 700+ scorer mentioned.

1. Record all mistakes on wrong questions and revisit them no sooner than 5 days. If still getting wrong, then zero on the subset's and go for more help.
2. Always read the explanation in the book to see why you get it wrong and take enough time to really understand the concept.
3. Pick a date and register. Don't study then register. There was a tendency to not take prep time as seriously.
4. Quite a few went through OG questions more than once. Not all questions on the repeat though; Mainly verbal sections and Quant that were areas of weakness.
5. Wrote practice essays at least 20 times under timed conditions.
6. Visit the test center before the day of test. Many did so a week or two before.
7. Time spent by most averaged between 3 and 4 months.
8. All averaged 2-3 hrs per day and no less than 10-12 hrs on the weekends.
9. Took advantage of every break during the GMAT. Take a bathroom break, some washed their face to wake up, got fresh air.
10. Study in blocks. One person mentioned studying in 80 minute blocks without getting up to build stamina. Another person mentioned studying in 75 minute blocks with exact numbers of questions that are on the real GMAT to build stamina and skill. Many agree that the ability to handle the time constraints is nearly as important is solving the problem. Scores in the actual GMAT are worse if you leave an answer blank than if you get the answer wrong.

Interesting note:
The Official Guide for GMAT contains questions that are in the 550 to 650 difficulty range.