I have been debating getting it for a while now, but 30 something quid is a lot to fork out for a game if I happen to not like it.
Are there quests and stuff, and are item drop rates any good/keep you wanting to play more? What is this whole AH thing? Do I need to pay a subscription to be able to use it etc?
There are quests. Basically the whole game is a quest following the story line. There are four difficulties and you basically run the same story line through each difficulty.
Item drop wise, it is very difficult to get the end game perfect gear. You basically have to roll multiple stat modifiers and each stat modifier has to line up to make it a perfect item. Personally I have stopped playing but if you are into grinding mobs repeatedly to get gear upgrades, then this game is for you.
The auction house is a place where you can buy/sell the items you acquire for either in-game gold or for real life money. The real life money aspect is optional and there is no subscription fee. If you wanted to, you would only have to pay for the game and that's it. (Unless they release expansions which they probably will)
From personal experience it's worth it till you reach the endgame content. The difficulties don't provide more stuff, just the same thing but tweeked to be a bit more difficult. Inferno itself is an endless grind, you really have to fork over hours grinding a certain place; in other word it get's boring quick.