What should I consider when deciding to whether or not to mine? I go here
http://www.alloscomp.com/bitcoin/calculator

I put in the amount of hashing power I am willing to purchase, EG: 100 GH/s. It tells me I will make $2,094.25 a month. I am aware that the exchange rate changes, difficulty rate rises. But what prevents me from making at least $1500. They say joining a pool is best, but even if I go it alone, what is the thing that will prevent me from making a reasonable amount of money based off of the information in this calculator. I cant be as simple as buying a miner and letting it run all day.
You really need to take into account some important factors, which that calculator is missing:


The current rate of increase in difficulty
The expected delivery of the mining hardware
The cost of the mining hardware
The cost of running the mining hardware (electricity cost)


I highlighted 2 of the most underestimated factors normally ignored by new miners.

Many mining hardware orders can have significant delays in shipping, due to high demand and low production volume. Sometimes you can wait months for a delivery, by which time the mining difficulty raised so high as to make the hardware unprofitable, even with the rise in BTC/USD price. This is especially true of new hardware pre-orders, be very careful about those (don't believe the expected shipping date).

I would recommend that you use this calculator instead.

Make sure you fill-in a realistic hardware lead time (1 unit for every 12 days wait).

Do your research otherwise you could end up not breaking even.