The legal permanent resident must live in the US, cannot work outside except in a few government jobs on temporary assignment, cannot vote in most elections, must apply for visas to visit third countries that citizens don’t need visas for, can be considered to have abandoned their green card status in only one day under certain circumstances and be refused re-entry, cannot petition for many classes of relatives and those s/he does petition for have a far longer queue to wait in, can be deported fairly easily, can be refused re-entry into the US as a public charge, and is excluded from much government employment, especially jobs requiring security clearances. But they can still be drafted, and if they meet the Selective Service criteria but fail to register as the law requires they may be ineligible for US citizenship and other benefits.
There are a few more also, but these are the biggies.
NOTE: If you want to add to this list, start your own answer please.
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