Here are a few I always bring up before rolling characters:

1. Policy on Feats, including any new, any banned, and whether VH is available as a racial option.
2. Policy on Multiclassing.
3. Policy on resting (i.e how often the group can expect to gain the benefits of a short or long rest.)
4. Policy on casting more than one spell in a turn. Save yourself the trouble and look this up now.
5. Policy on stuff like Critical hits, being knocked unconscious, death saving throws, resurrection, etc.

There are probably more but I just woke up. Also a few specific tweaks I run:

1. Eldritch Blast scales with warlock level not character level
2. Divine Smite takes a bonus action to activate
3. No SCAGtrips
4. Initiative keys off Intelligence instead of Dexterity

Again, probably a few more but -yawns mightily-