I'd like to use this pin to just read a simple TTL (+5v or 0v) input just like I'd do with any other pin. Does this "open drain" mean it'll behave strange?
OK, open drain has to do only when a pin is used as an output. When you want to use the pin as an input then you are fine, the pin works completely the same as any other input pin.

Also, what if I want to use it as an output and drive something like an LED?
It can work as an output, but only as a switch to ground (as Russ's pic correctly shows). If you connect your LED from + power thru an appropriate resistor then to the open drain pin all will work fine.