The original match making system was based on a rating and points were gained for wins. The rating system was matching based on levels for the most part. i.e
Tier1 - Level 15 rating < 1470 & Level 14 tier3
Tier2 - Level 15 rating 1470 to 1530
Tier3 - Level 15 rating > 1530 & Level 16 tier1
Basically there's an overlap for low rated players with the high rated of those a level below, and high rated players with the low rated of those a level above.
How rating changes...
yourTier < enemyTier - rating + 5
yourTier == enemyTier - rating + 2
yourTier > enemyTier - no change
yourTier < enemyTier - no change
yourTier == enemyTier - rating - 2
yourTier > enemyTier - rating - 5
Once level 20 was obtained the longer level 20s had more advantage over newer level 20 so tier system was eventually integrated.
See early level 20 tier's for detail.