-the free stress.
Which is made even worse by the strong reduction of unstressed vowels in Russian, thus it is more difficult to write down a word from hearing. Personally, I prefer orthographies that mark stress explicitly like Spanish or Greek.
Polish is more "average European" than Russian in some areas, e.g. in Russian you usually say "Y is at X" in the meaning "X has Y", while Polish is similar to English in this respect. Polish has a normal verb
musieć "must, have to" while Russian uses some impersonal verbs or specialized adjectives (должен; however, Polish
powinien "should" is quite similar). Polish adjectives have a typical superlative form, which is very reduced in Russian. Russian uses a zero copula while Polish uses the verb
być "to be" or the particle
to.
OTOH, the areas where Russian is more analytic may be perceived as easier (for instance, in the past tense there's no person marking on verbs).
Russian also has less gender marking in the plural, but Polish has the virile and non-virile distinction (or masculine human and non-masculine-human) on nouns, verbs, pronouns and adjectives. Russian seems to distinguish animate and inanimate nouns in plural only in the accusative, but also among feminine nouns, while in Polish only masculine nouns are divided into virile and non-virile. Here Czech is even more complex, because it distinguishes masculine, feminine and neuter forms in the plural. Also, as someone's said already, in Polish the genitive endings of masculine nouns are somewhat hard to predict, but some fluidity is acceptable, particularly in the colloquial language.
Polish conjugation may seem more difficult, but it's partly due to the way it's usually described. In the Russian nomenclature some types of conjugation are lumped together while in Polish they're described as separate conjugations.
Polish appears to have more consonant and vowel alternations than Russian, for example in Russian you have Nom. рука, Dat. руке and in Polish
ręka, ręce respectively. In Russian the alternation k : c has been levelled. Since in Polish the former soft /r/ became /s`/ or /z`/, there are alternations like
trę :
trze "I rub, [he/she] rubs"; in Russian just the palatalization or lack thereof changes. Polish also has apophony like
lato : lecie (Russian: лето, лете).
Russian has two locative cases for some words, depending on the preceding preposition. OTOH, Russian doesn't have a vocative case.