Halving is definitely a big event in bitcoin history. The reduction of new bitcoins generated when a block is mined, the decreasing in bitcoin supply. If you are miners then you should worry about the decreasing bitcoin reward for each block, the profitability, and much more. For most of us not miners, we just worry three things, the price of bitcoin, the speed of comfirming a transaction(Hashrate) and the new supply of bitcoins in circulation. The bitcoin price is still volatile amidst the halving and maybe could rise sharply or decrease. No one knows what will happen but there is not much of changing at the 2012 halving, maybe rise? Maybe stagnant? Maybe decrease?
For the speed of transaction, the halving may cause some small mining companies the lose their revenue and shut down their hashrate. Currently 1.5 exahashes of hashrate, it will certainly decrease with the event of halving and the other miners struggle to survive the decrement of new supply of bitcoin. In the other hand most of us will trade much and it may lead to overflowing of bitcoin transaction and confirming time  increases should they shut down their mining rigs.
The time for bitcoin halving is 2 hours away as we speak so good luck and hope the bitcoin price increases for the best for us users and miners....