Here is a list of the main problems, I think.
1. Can't build a wooden boat that big without it falling apart.
2. Not enough time to gather all the animals.
3. Not enough room for all the animals.
4.Not enough water.
5. No accounting for different erosions of present mountains, i.e. why don't the Rockies look like the Appalachians?
6. Impossible to maintain polar ice caps or regrow them in time period since flood.
7. No sedimentary traces on sea floor.
8. No evidence of flood in ice core samples.
9. No evidence of flood in tree rings.
10. Order of fossil sorting inconsistent with flood, e.g. ancient flyers, such as pteranodons, are in older layers than modern sloths.
11. Surface features preserved in rock layers, such as rain drops, dunes, and so forth. How could that happen with a flood?
12. Sedimentary layers too keep for a single flood, unless it laid down miles of sediment per day.
13. Varves. Period. Varves alone blows the whole thing out of the water, so to speak. Ask me ifyou don't know what they are.
14. Layered fossil forests. How do you get one forest to grow on top of another during a flood?
15. Limestone.
16. Chalk.
17. Layers of solid salt.
18. coral layers.
19. Too many fossils to have all been alive at the same time.
20. Too much coal and oil for the forests to have existed at the same time--it would be bigger than the earth.
21. How did ground plants survive being under water for months on end?
22. How did freshwater, brackish water and salt water fish all survive?
23. What about short-lived species, like may-flies?
24. Why are there kangaroos in Australia, but not Arizona, and roadrunners in Arizona, and not Australia? e.g. how did koala bears get from Armenia to Australia, and why did none of them stop on the way?
25. Why no records of the flood by Egyptians and even Babylonians?
Thanks to
Mark Isaak