Can anyone explain landleases to me? I've owned two coops in the city, but never a
condo, and am looking at a building with a long landleases. What are the legitimate problems with landleases?

More than anything else it is really high monthlies that are NOT tax deductible -

I think of it as renting a plot of land & then building a house on it. It makes no sense to me to build one's house on a rented lot.

From the landowner's perspective, it allows you to retain ownership and get a stable revenue stream. Typically the rent is reassessed periodically (e.g., every five years) to reflect the current value of the land. Then, at the end of the lease term (less than 101 years) you have the option of either tearing down whatever was there and making better use of the land, or entering into a new lease with the tenant.

"Typically the rent is reassessed periodically (e.g., every five years) to reflect the current value of the land."

Does the landowner have carte blanche to raise the rent to whatever he wants? Or is there a quantitative formula of some sort that determines what the increase (or decrease) will be?

Otherwise, it would seem extremely risky for the condo buyer.

Except for Battery Park City, Roosevelt Island, QueensWest, and Brooklyn Bridge Park (whatever that is) leased land can't become a condo. A co-op corporation, however, can either own or lease.

The periodic reassessment is done by an impartial appraiser or other third party, so theoretically neither lessee nor lessor can run roughshod over the other.

If your landlease resets during a bubble, your ground rent will likely be reset at a rate that will be unsustainable when the market falls.

which building?