1. What they can contribute to the betterment of the US ("diversity" isn't a contribution).
2. What they believe (their philosophies, morals, ethics)
3. Their motivation for coming here
4. The rate of immigration
If they are coming to do harm, to leech or are diseased, they are a direct threat and must be kept out. If they don't speak English, have non-Judeo-Christian philosophies, come from cultures with radically different customs, are illiterate, are physically or mentally incapacitated, etc., they present a longer-term threat IF they come at such a rate that they cannot be assimilated. Ice chips dropped into a pan of boiling pasta can be assimilated without notice, but if introduced at too high a rate they will cool the water and ruin the pasta.
Immigration to the US is not a right. America's will must set the rules—not the UN or anyone else—or the great, constitutional republic will become just another mire.