What is semantics behind BJT and MOSFET transistor symbols?
BJT:
the arrow represents the base-emitter junction, which acts as a common diode (remember the 0.7 V drop). So the arrow is like the arrow in the diode symbol.

MOSFET:
This dates back from the JFET days: in that case there's a gate-channel P-N junction, and the arrow points from the P to N doped silicon.
The three short lines indicate that it's an enhancement FET: they symbolize that the channel doesn't conduct when the FET isn't activated. In a depletion FET that will be one uninterrupted line, indicating that the channel does conduct when not activated; the FET is switched off by applying a gate voltage.
The line at the gate side symbolizes that the gate lays over the channel, forming a capacitor (a FET's operation is based on capacitance) with it.