Looking back at this list, most of this seems pretty obvious. But as the say, hindsight, 20/20, yadda yadda.

• End grain looks crappy. Unless it's a special design element, make your joints in a way to hide or dress up end-grain. If I did this table again, I'd make the bottom brace longer so that the end grain was hidden by the legs.
• Check for grade and vendor marks on the wood and remove them. And be sure to check for marks before you coat it in poly. I didn't notice them until they were coated and it was too late by then unless I wanted to strip the table. Not really something I wanted to do, so you can see the grade marks on some of the legs.
• Use a dropcloth. Or something, anything really, to protect anything you don't want coated in polyurethane. My patio now has some very well protected dime-to-quarter-sized circles, which I haven't quite figured out how to get rid of, because of drips from the table.