Overwatch Loot Boxes – Are There Exclusive Rewards?

According to wiki,

Each Loot Box contains four random items that can be used to customize the appearance of heroes. Items come in common, rare, epic, and legendary quality levels, and every Loot Box includes at least one rare quality or better item.

and,

even unopened Loot Boxes will glow in a color corresponding to what items are inside.

so if you haven't gotten a box before with a different glow, you just were unlucky that you hadn't gotten as rare of items in your previous loot boxes.

There are no published drop rates as such but here is a link to a Reddit thread which has compiled a list of percentages over 1000 crates by various people compiled into one list. It's still a small sample size in the grand scheme of things but it's better than individual stats. To follow the links below you can use the Reddit thread.

Video link          Common  Rare    Epic    Legendary
Force               235     125     31      9
Royal               113     66      15      6
Futile              116     67      15      6
Jeel                118     64      14      4
Boogie2988          117     64      18      5
DrFeelgood          121     60      13      6
Masier              117     64      14      5
MFPallytime         237     132     27      8
Stylosa             245     125     28      6
Drift0r             116     58      15      7
Redmercy            234     125     33      12
Nero                238     121     29      8
Outfoxed            284     167     31      14
Gamespot            121     65      14      4
Total count         2412    1303    297     100
Percent of Total    58.66%  31.69%  7.22%   2.43%
Drop rate           97.08%  78.22%  25.91%  9.38%