Well, I attended the Prophecy Pre-Release on Saturday and had a great time, placed 8th and won 4 boosters and then managed to pick up 6 rares in a booster draft afterwards. My first impression of the Prophecy cards is that they are very good, better than Nemesis and infinitely better than Masques IMHO.
There are quite a few cards that cause you to tap all your land, or have differing powers dependent on whether you have land tapped or untapped. There's also a lot which require you to sacrifice lands to power them so I think Groundskeeper is going to become popular and we are going to see the average land count in a deck increase.
I managed to get some interesting rares:
Avatar of Woe, 6BB, 6/5 Creature
If there are 10 or more creature cards total in all graveyards, Avatar of Woe costs 6 less to play.
Avatar of Woe cannot be blocked except by artifact creatures and/or black creatures.
Tap: Destroy target creature. It can't be regenerated.
Infernal Genesis, 4BB, Enchantment
At the beginning of each player's upkeep, that player puts the top card of his or her library into the graveyard. He or she then puts X 1/1 black Minion tokens into play, where X is that card's converted mana cost.
Elephant Resurgence, 1G, Sorcery
Each player puts a green Elephant token into play. Those creatures have "This creature's power and toughness are each equal to the number of creature cards in it's controller's graveyard."
Forgotten Harvest, 1D, Enchantment
At the beginning of your upkeep, you may remove a land card in your graveyard from the game. If you do, put a +1/+1 counter on target creature.
The plan is to use the Infernal Genesis to generate the conditions for me to get the casting cost discount on the Avatar of Woe. The Elephant Resurgence will hopefully give me a big fat creature - I'll have to try and kill my opponent's Elephant though (Terror would do it - or Repopulate *after* I have my Avatar in play). The Forgotten Harvest is to make sure any discarded lands don't go to waste. Add in a bunch of graveyard recursion cards and there's the basis of an entertaining deck!
On a different note, I spotted a possible combo that would prove effective against those (annoying ;-)) blue mages who sit back with all their mana untapped waiting to Counterspell during your turn:
Citadel of Pain, 2R, Enchantment
At the end of each player's turn, Citadel of Pain deals X damage to that player, where X is the number of untapped lands he or she controls.
To ensure you don't get stung by the Citadel, play it in conjunction with:
Troublesome Spirit, 2UU, 3/4 Creature
At the end of your turn, tap all lands you control.
As an added note, there are a few red creatures which are pumped up if all your lands are tapped which would also benefit from the Troublesome Spirit such as Branded Brawlers, Spur Grappler, Keldon Beserker.
If anybody has any additional ideas, especially for the green/black graveyard deck then I'd love to hear them. Please note that these are only my initial thoughts based on the Prophecy cards I have, I'm waiting for the unofficial spoilers to become official Wot lists before I start to consider what other cards might go well in the decks.