It's simple ... The biggest problem is Profit !
Things like Electric costs and the continuesly raising mining difficulty will decide wether a coin is still profitable.

Also, if you have equipment already laying around that could reduce your return on investment.
You also need to be on top of your game and switch between coins as the most profit could easily change during a week.
If your starting out now and got some money to spend i would invest that right now in coins that are at a low. These past few days were perfect for that.
Within the next week you can make more profit than you could with mining for a couple of months.