Most sets of Magic feature a certain kind of game mechanics, and favor some archetypes more than other.
For instance the previous set (Return to Ravnica, Gatecrash, Maze I don't remember) favored multicolored decks, even 3 colored ones, featured lots of multicolored creatures, whereas the beginning of the current set (Theros, Born of the gods) favored mono colored decks. Some capacity are absent from different sets, and some sets really favor some kind of build (eg Theros set gave the opportunity to create really powerful blue aggro, Innistrad set allowed very efficient graveyard oriented decks...).
Nevertheless, some archetypes are always valids, and even if some blocks don't favor them, you will still find useful cards and maybe the next set will favor it again. Some archetypes are also more common and nearly always efficient, "red deck wins", a mono red archetype is among them.
If you don't care about having a deck in the current set, you shouldn't mind about this, if you seek competitive T2 game (last two blocks), you will change deck every block to have the more favored (and fun) archetype. An easy way to see toward what a set is slanted is to have a look at the top 10 tournament decks, you can also guess that by yourself after gaining some more experience by just looking at the cards featured.
To finish, if you play drafts (which I advise you, it's really funny), the favored archetypes will be different from built decks.