Essentially resetting your deck (and your opponents) while leaving the board state as it is - a neat trick, and one that someone paid $84,000 for
Play Boosters are great for building powerful decks, fun to open, and are balanced for Limited play
The first ability to pump another creature you control pairs perfectly with Elvish Mystic, but even if you dont have an ideal draw, you can always play another creature before you attack so that the counter has somewhere to go
Finishers/ Surprise /Alternate win conditions Herald of Secret Streams make creatures with +1/+1 counters unblockable and is a great card for any blue +1/+1 counter deck
If you consider the fact that a foil Urzas Legacy Memory Jar costs well over $700, buying a tournament legal foil copy featuring the same artwork for $90 is a bargain
Tezzeret, Cruel Captain Tezzeret, Cruel Captain gets major points for a few reasons