So 1.9 Pre4 apparently activated the enhancement system so that stuff actually has effects and levels now. Something like this:
Silk Touch allows you to mine ore blocks (like the actual block, so instead of getting iron ore you get a placeable iron ore block for example). They have enchantments that cause knock-back, enchantments that catch enemies on fire, ones that deal extra damage to spiders or extra damage to undead, water breathing on armor; not to mention increased durability, strength, armor, etc.
Now I feel shitty though, in my SSP world I was doing a great job of staying alive and had 312 level points. I literally hadn't died at all yet. Last night I ventured into the
new Nether for the first time so I could get a Blaze Rod... I didn't realize the Blaze Spawner would spawn three Blazes at once and they fried me. I was able to run back and get all my items and farm some rods but now I'm like level 20 again :'(. I'm going to have to farm Slimes now so I can get some Fire Resist potions.