Most players will agree (maybe??) that open tunings make SOME things easier to play. Some things are practically impossible without some non-standard tuning. Worth exploring for sure.
As far as really defining "open tuning", the term implies tuning to a simple major or minor chord. S&r makes a fair point: standard tuning (Em7sus4, or maybe more exactly Em7add4 or Em7(11)) is just a hair away from being an "open" tuning.
Tunings like DADGAD are more correctly called "alternate tunings": not standard, and not basic maj or min chords.
"Drop" tunings retune the 6th (or 5th & 6th) strings only, leaving the rest in standard.

Chris C. (member, not on my computer)