SEO: Google links exchange

I read many articles saying about Google would change its algorythms for evaluating Page Rank and links exchange will not work so weel as now.
the main idea that Google will ignore or ban the links to my site if the link is not inapriate .
i just think about simple example:
i have 2 sites which are far from each other about topics.
One site sells aquariums and another site is site of web studio.
and Web studio site has link to Aquarium site as part of its portfolio.
The site about aquariums has inbound link from web studio site but the whole site of web studio says nothing about aquariums.
is it considered as a bad link?

I guess how they will distinct bad and good inbound links ?
or it is another tricks from google, when they say something about improvement and big ideas but it works as usual..