For basic low frequency work vias are pretty much "meh". The only thing you need to watch for is when running larger currents - the vias tend to have a higher resistance, so dissipate more heat.
However, the interesting things start happening at higher frequencies.
Vias start to become antennae. They can radiate EMI like nobody's business. So for high frequency signals, keep the vias to a minimum.
And by "high frequency signals" I don't just mean if you're intentionally working on RF systems. Digital systems have some very high frequency components too. For instance, a 10MHz SPI bus - the clock is running at 10MHz, with harmonics at 30MHz, 50MHz, 70MHz etc (depending on slew rate of course).
Also you have the question of impedance to look at. A via has a different impedance than a normal trace, so if your circuit is impedance sensitive (if you are doing impedance matched traces, PCB antennae, etc) then you have to take the vias into account in your calculations.
So for general power distribution, vias should be noted and thought about. For high frequencies, vias are anything from frowned upon to down right no-nos.
Everything in between is pretty much irrelevant.