there are some big differences and some small ones...
-there are a number of great plugins that won't work on standard
-there are some things like the motion tracker that aren't in standard
-certain keying filters and masking options aren't available
stuff you won't notice as much...
network rendering and a few other features.
i've used both and it's hard to tell the difference...unless i need something i don't have anymore. the motion tracker is the biggest one for me. i can't afford AE Pro for now so i'm working with standard. unless you need some of those few differences, then it's not worth it.
also, i'm sure there are a lot of things i can't think of right now. so this is definitely not a difinitive list