Electronic – Datasheet Parameter for Drive Strength

Do not trust an MCU output pin to effectively limit the current. It will likely damage your part. Even if it does it correctly once, or the first few times, or even the first few hundred times, you are compromising your design. You need either a higher current limiting resistor (your LED may turn on with 2 or 3 mA) or you need a buffer.

You can send a signal on to many many other destinations (the number of destinations is called the "fanout" of the signal). The more destinations it goes to, the longer your critical timing path potentially becomes though, so the fmax of your design may suffer.

The tools will usually replicate the logic that drives those many nets if the timing becomes slower than you have requested in order to try and meet your timing target.

For Xilinx, this list of appnotes offers advice for reducing fanout when this is on your critical path (http://www.xilinx.com/support/answers/9410.html)