- complex structures (not necessarily always 4/4 or 3/4)
- variations in dynamic range (loud parts vs. quiet parts)
- storytelling / philosophical themes (the human condition, freedom, suffering, literature)
- intense "solos" (Mendelssohn insanely fast yet moving segments on the piano vs. Petrucci going crazy on the guitar)
- mastery of musical instruments (particularly true in progressive metal, but metal drummers, guitarists/bassists and keyboard players tend to play at a very high technical level)
- song length (like Seventh Wonder's "The Great Escape" at about 30 mins)
None of these individually are unique to either genre, but taken together, we can start seeing quite a bit of an overlap.