Why the "unconditional discharge"? I can understand no jail time, and no home confinement, but why not at least sentence Trump to a significant fine? This was a campaign finance charge after all, isn't it?

It wasn't a campaign finance charge - it was a business records charge.

I'm not a criminal lawyer, but I would hazard a guess that the judge didn't impose a fine because doing so would subject Trump to the continual threat of jail, which might give appellate courts pause. Again, I know little about NY criminal law, but I expect that when a criminal fine is imposed, the court retains jurisdiction and oversight over the defendant to make sure that the fine gets paid. And if it doesn't get paid, the court can order penalties. And one of those penalties is probably jail time.

If Merchan imposed a fine, Trump would never pay it. I know that, you know that, and all the appellate courts and SCOTUS justices know that. Which would mean that Merchan's court would have to be able to summon him back for further proceedings, with the possible imposition of jail time....which would keep all these weird and thorny legal issues in everyone's lap over the next few years.

So he didn't bother. There's no fine he could realistically impose that would have any impact on Trump (who is now worth many many billions, and could easily get someone else to pay any reasonable fine). So why open up the legal can of worms?