I am Czech/Slovak (parents from both countries) and I can best understand Polish, then Ukrainian and Russian and only then Slovenian and Serbo-Croatian.
The reason for that is that Czech Republic, Slovakia, Poland, Ukraine, Belarus, and Russia were always in one "pack" while Yugoslavia was separated by Hungary and Romania from the others. Therefore you have more Italian words in Slovenian and even more "exotic" in Serbo-Croatian.
And it's because of the arrival of Hungarian tribes in around 900-1000 A.D. to this region. They separated these Slavic tribes and therefore the different "outcome" in the end. Of course, I am generalizing this geo-linguistic issue, it is much more complicated in reality.
@Dominik Lukes:
Bluntly, Slovak and Slovenian have nothing in common other than being both Slavic languages.
Sorry Dominik, but this statement is completely wrong!
We have a lot of common words and even more word-roots.
Some examples:
EN | SK | SL
----------------+------------+------------
Slovak language | Slovenčina | Slovenščina
(he/she/it) is | je | je
to talk | hovorit | govorit
car | auto | avto
chin | brada | brada
function | funkcia | funkcija
eye | oko | oko
arm | ruka | roka
finger | prst | prst
brother | brat | brat
sister | sestra | sestra
newspaper | časopis | časopis
window | okno | okno
Btw. check this ([y and i] and [ä and e] are pronounced basically the same in Slovak):
one | jedna | ena
two | dva | dva
three | tri | tri
four | štyri | štiri
five | päť | pet
six | šesť | šest
seven | sedem | sedem
eight | osem | osem
nine | deväť | devet
ten | desať | deset
If you still want to say that "Slovak and Slovenian have nothing in common other than being both Slavic languages" after these basic number comparison showcase, you are simply being ignorant to the facts.