Simply as the title says (but not only negatively!).
Example:
Very cold area:
1) -10% movement speed
2) no basic health regeneration
3) -10% attack speed
4) +20% armor
5) flying mounts unavailable
Very warm area:
1) +20% energy/mana cost
2) +10% attack speed
3) +10% damage taken
4) +20% bleeding debuff duration (on self as-well)
Swampy area:
1) losing 1% hp every 10 seconds (ground contact only)
2) no health regeneration
3) -10% healing received.
and so on, many more ideas can be easily invented. They sounds real anyway.