I believe that top 10 is a bit of too much of a request. Maybe top 3 would do it better.
No special order, except that negative keywords deserve high placement for sure:
1. Negative keywords - Use search terms report to find out keywords that are making impressions without clicks or clicks that do not convert and add them to negative keywords list. Also use the keywords tool to find if there are any that you would pout on your negative list.
2. Negative locations - if there are locations you know you can't serve, exclude them.
3. Keyword level tracking - if possible, track your conversions on a keyword level. The best would be to have a system that connects cost and earnings in a single report, so you can see right away (without export/import/calculate) how your keywords are doing.
4. Ad CTR - check CTR of your ads. Over the time, you may learn how various situations affect your CTR (specific ad text, competition, time of the year, month, week, etc.).
5. AdWords Preview Tool - This may help you see what's happening in certain locations, with various language settings and G domain extensions (countries).
6. Report offenders to Google - depending on the industry, there may be intruders that do something against policies. Report them.
7. Use some sort of analytical software - Most use GA, but you may use something like Piwik as well, if you're not willing to give more data to G than you have to. Setup PPC links properly so you can distinguish PPC from other traffic.
8. AdWords Editor - Super duper AdWords tool.
9. Distinguish mobile from regular campaigns - Well, this will change when we all get forced to use Enhanced Campaigns. Still, I believe it will be possible to "kill" mobile with low bids if what you offer is not going well with mobile based visitors.