What are the most notorious things for which Russian is known as one of the hardest to learn?
Alphabet. It's not Latin, people spend a week or so to only remember that P is not [p], but [r].
Aspect. It is formed irregularly, it has tons of minor meanings, and the exact interpretation depends on certain properties of the verb meaning. And it also depends on prefixes.
Inflection. You have to learn large paradigms only to be able to say a short sentence.
Rich derivation. You can form many words just by adding suffixes.
Word order. It is not free, as some people say, but it encodes the information structure of the sentence.
Phonology. Quality of vowels changes depending on the stress in the word (which is also not always fixed), voiced consonants become voiceless depending on the following consonant or on the position in the word.
Finally, Russian is not as hard to learn as Welsh or Hungarian. But not many people want to learn these languages (for some reason), and that may be another reason why they focus on Russian and say it's hard. It probably wouldn't be so hard in comparison =)
If you mean the difficulties for a native Russian speaker, they are as follows:
Comma placement rules
Whether participle не- is written together with the following word or not.
Whether н is double in suffixes (choice between нн and н).
Some people experience troubles in choice between -тся and -ться although this is not a problem for myself.
- Inflections, especially the cases and verb conjugations. Also the number and gender.
- Pronunciation of certain sounds non-existent in languages like English. That includes the vowel sound of Ы [ɨ], all palatalized consonants, and rolling the Р [r]. Also, the chains of consonants as in "*вздр*огнуть".
- Stress in words. It's mostly unpredictable in Russian and you'll have to refer to your dictionary a lot.
You can easily yandex it up yourslef, as follows:
- go to yandex.ru, which is better than google for Russian searches
- Search "типичные ошибки в русском языке у американцев"
Wikipedia says" According to Yandex marketing, one of its biggest advantages for Russian-language users is ability to recognize the Russian inflection in search queries