Legendary Questline so far just gives you a Gem (+500 Str/Int/Agi) for a "Sha-touched" weapon (found in LFR or Normal/Heroic raids), and adds a Prismatic Gem to it for the next part.
In 5.2, it continues onward to give you a Meta Gem.
Commendation System lets you purchase commendations for Pandaria Factions at Revered that give you 2x Rep for all characters on your account for that faction. Useful for alts and getting to Exalted.
5.1 also added some new factions and changed Krasarang into a bit of a PvP Zone. Worth doing the new factions since they have some actual story behind them and it's pretty good. Also added Item Upgrading (removed in next patch), more scenarios, and Brawler's Guild, which is sorta like a Fight Club-type thing.
As for 5.2, read this:
http://us.battle.net/wow/en/game/patch-notes/under-dev
That'll give you a short overview, but it's obviously not everything.
Here's the full 5.2 PTR Patch Notes so far:
http://us.battle.net/wow/en/blog/8226552