Eclipse-like comment formatting in IntelliJ IDEA?

The closest thing that you can get is Edit | Join Lines (Ctrl+Shift+J). You have to select the lines you want to join first.

To wrap long comments enable Settings | Code Style | JavaDoc | Wrap at right margin.

For Javadoc comments, you want to make sure the "Wrap at right margin" setting is checked. See Code Style > JavaDoc, under "Other". However, this setting only seems to take effect when you reformat the whole file, since a reformat of just the Javadoc (i.e., select the Javadoc, then do a Code (menu) > Reformat Code... or CtrlAltL) that exceeds the right margin doesn't force it to wrap. If I reformat the entire file, then it wraps at the margin as expected.

This seems like a bug (though one that doesn't seem to have been reported), since if you have to set the "Ensure right margin is not exceeded" checked, then selecting the Javadoc text and doing a reformat code does indeed wrap the lines. This setting is in Settings > Code Style > Wrapping and Braces. You can also do a search in the Settings dialog for "ensure right margin".

You'll still have to manually join the lines using CtrlShiftJ

This might be worthy of an improvement request to JetBrains.

Existing comment will be reformatted when you do "Reformat Code" (⌥⌘L in Mac).