1. Reached alpha level cap of level 29.
2. Leveling became "tougher" after level 20 with having a grind more and more each level(aside from BG's+dailies). (not dissing grinding btw, i love it)
3. Level 15 opened up Janus I believe, and introduces you to an awesome PVP battleground.
Level 17 you finally get a Mech, a very crucial machine for anyones survival and longevity in PVP and even PVE.
Level 23 you can finally enter Caergate, an amazing pvp/pve zone with each faction residing in each their own corner.
In Caergate there are also Sub-Bosses surrounding the center of the map, with a Main Boss named Titan right in the middle. (Have a good team when trying to fight these subs/main boss)
Caergate is also where you can take part in an escort quest, was very fun
4. I did not create an alternate character.
Side Note: At level 20 you could also equip a rare mech, but you have to find one first