— Terminals at right angles to the stroke. http://c.jon.gd/image/3Q0y2u323j3C . Arial looks particularly sloppy with jaunty terminals. It is possible to have a similar grotesque sans-serif feel with offset terminals (see Univers & Akzidenz Grotesk) but they're a crucial part of what give Helvetica its character.
- The uppercase R. Has a really strong leg compared to Arial's half-assed flaky leg.