It's pretty obvious.
He lies often. Not just little fibs; huge, glaringly obvious lies that are not only easily disproven (often with a quick Google search), but often cause real damage when people believe them. He parrots QAnon conspiracy junk. He makes even Nixon seem honest.
He's overtly corrupt. He abuses public offices (like the DOJ and USPS) for personal gain, and he even admits that what he's using them for. You know how he was impeached for trying to squeeze Ukraine for dirt on Biden? He acknowledged it. The only reason he wasn't convicted was because the Republican-held Senate covered his ass.
He panders to xenophobes and racists. He scapegoats entire cultures and foreigners as a whole. He also has an almost comic inability to condemn racism. When people found that Stephen Miller consorts with white supremacists, the Trump administration refused to fire him. He shifted law enforcement attention away from right-wing racist extremists despite them being the primary source of terrorism in the US. And then there's that softball question about condemning white supremacy at one debate — it was the easiest question on the planet, but he couldn't bring himself to say "I condemn white supremacy" until he was forced to afterward.
He rejects science. Human-made climate change is a well-documented thing with a broad scientific consensus, but Trump pretends it's not real. He's antagonistic with health experts who are just trying to save lives. Hell, this is the man who looked directly at a solar eclipse after being told not to. We should have political leaders who respect science and defer to experts in a given field.
Trump inherited a thriving economy from Obama. You can give Trump a bit of credit for not screwing things up pre-pandemic, but even then he added volatility to the market that didn't exist before. Also, low unemployment isn't the only factor. You need a higher quality of life and better opportunities. What good is low unemployment if you need to work two or even three jobs to avoid poverty, and never have a hope of affording college? Let's not forget that the Trump/GOP plan ultimately favors the rich, not the broader public.
On foreign policy, what progress there's been has been undermined by Trump in other areas. He loves authoritarians like Putin and Kim (both for economic opportunities and because he cherishes unquestioned rule). He goes soft on them when they have no intention of caving in. He weakens relationships with important allies outside of Israel. And those authoritarians not only know this weakness, but exploit it.
That Trump is also a petty, vindictive asshole on Twitter is just a tiny drop in a much larger bucket.