Double-click with Direct Selection to enter Isolation mode.
You can only enter Isolation mode by double-clicking with the Selection tool. Why doesn't the Direct Selection tool work for this? Or the Group Selection tool, for that matter? Was there ever even an explicit design decision made that only the Selection tool should be able to do this?
I do the vast majority of my selecting with the Direct Selection tool, and about 50% of the time when I want to isolate something I just try double-clicking on it with the arrow-shaped tool I'm currently using, and grumble because it doesn't work, and hit the hotkey for the unloved Selection tool and double-click and do whatever I wanted to do.

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Double-clicking with Group Selection tool selects the parent object :) This is one of main functions of the tool! Each new click selects a parent one level above.
But Direct Selection tool does nothing with double-click indeed. That’s why I prefer to use a hotkey to isolate. More of this, I now use Rick Johnson’s Toolshed now to isolate. It allows to have a hotkey to go one step up in isolation, to have a dedicated key to end it (since Escape does too many things), allows to isolate text, to isolate nothing...
These are requested, of course:
http://illustrator.uservoice.com/forums/333657/suggestions/32403778
http://illustrator.uservoice.com/forums/333657/suggestions/44124621
http://illustrator.uservoice.com/forums/333657/suggestions/35809273Voted.