I found Russian pretty straightforward, though I didn't get really far when I studied it. The grammar was fairly straightforward. I took a little Arabic and did okay with it as well. Chinese and Japanese are killers for me, especially when you start talking about tones. I wish we'd all switch to one language and just stick with it.
You obviously didn't get very far in Russian. The words straightforward and Russian don't mix. Like other European languages, Russian nouns have genders which can be masculine, feminine or neuter. Not only do adjectives change endings based on gender and case but also nouns, yes even proper nouns like a person's name! This is extremely difficult when you are first learning Russian but you slowly get used to it. The good news is Russian has no definite or indefinite articles and word order isn't as stringent like in German. Forming the past tense is really easy too. However there are other really nasty things to deal with
(1) prepositions that have multiple meanings (2) an endless amount of motion verbs to express that you are going somewhere
(3) imperfective vs perfective verb aspects which essentially means you have to memorize two verbs for every one. (4) counting in Russian is a bitch with 3 different endings based on the amount.
(5) stress changes when conjugating some verbs
I'll give you some easy examples to get the point across. There are six cases in Russian: nominative, accusative, dative, prepositional, instrumental, genitive. Nominative case is the easiest. This is probably where you stopped.
Nominative
An adjective like хороший(good) is used for masculine nouns but changes to хорошая for feminine, хорошое for neuter and хорошие for plural.
он хороший актёр. He is a good actor
она хорошая актриса. She is a good actor
они хорошие актёры. They are good actors
masculine adjectives have 3 different endings in the nominative but just showing you one. And don't get adjectives mixed up with adverbs like хорошо(well) as in он говорит по-русски хорошо.(he speaks Russian well)
Accusative.
feminine nouns change from а to у and from я to ю. Adjectives also take on this change. I.e in nominative the adjective русская is changed to русскую in accusative, but also any feminine nouns like литература.
я люблю читать русскую литературу. I like reading Russian literature.
вчера он читал книгу. Yesterday he read a book. (also example of past tense formation. читать becomes читал/читала/читали depending on gender of subject)
The good news is that neuter nouns don't change and masculine only change when using animate nouns like a person or animal.
Dative.
Ohhh my, you thought your name was Ivan(иван) or Anna(анна)? But if you give something to Ivan or Anna guess what? Yea their name changes endings too!
он дал ивану книгу. He gave a book to Ivan
она дала анне книгу. She gave a book to Anna.
At this point Russian isn't too hard but then you get into the really tricky grammar of the other 3 cases especially genitive and instrumental. Genitive isn't just used for possession but for quantities as in
nominative: это вода. This is water.
accusative: я хочу воду. I want a water. To make it worse the stress changes from the last syllable of water to the first
genitive: бутылка воды. A bottle of water. And the stress changes back to the 2nd syllable of water.
And for counting it really gets ugly. Take the word for hour(час). One hour is час, 2-4 hours is часа and 5-20 hours is часов. One ruble is рубль, 2-4 rubles is рубля, 5-20 rubles is рублей. So 114 rubles is 114 рублей and not рубля
Now for the fun stuff! Prepositions, motion verbs and aspects. Russian has a mind dizzying amount of verbs to describe that you are going somewhere and it is based on (1) one way or unidirectional (2) by foot or by vehicle and to make matter worse a bunch of prepositions to express going to a place. To make it even harder, each preposition has multiple meanings. I.e на can mean to, in, at, for. Same for В. i.e
я иду на работу. I'm going to work. (by foot)
я еду в магазин. I'm going to the store (by vehicle)
Yet you can use на and в to also mean in/at as in
я на работе. I'm at work
я в магазине. I'm in the store.
Noticed the noun change? This case is is prepositional so again the nouns and adjectives change ending
Aspects are used to express completion vs a continuous action. Sometimes you just have to prepend по or про or при to the verb but it is not always that easy. For example to express that you read a book yesterday but didn't finish it you'd use the impefective version
я читал книгу
But if you read it and finished it then you'd use the perfective version
я прочитал книгу
The problem is that not all verbs are this easy to form aspects. Sometimes the verbs are completely different so requires a lot of memorization.
Anyway, I barely touched on the grammar. Russian grammar gets very complicated and has A LOT of exceptions. But hey the tradeoffs are no articles and past tense is really easy. So not bad afterall eh?