For instance, Poland lost the COMECON markets and didn't have the benefit of a common language with a huge trading partner next door.
I think any reasonable person, if given the choice, would choose to border Germany (most importantly), the Baltics, the Czech Republic, and Sweden (across the puddle)... than Russia, Belarus, Moldova, Hungary, and Poland - no? So I wouldn't chalk up sharing a border and a language with Russia as such a big advantage. In fact, I think these days very few Ukrainians see bordering Russia and sharing a language with it as a great advantage. I agree, however, that it was "location, location, location" that predetermined the present disparity between the two countries.
Ukraine of the 1991 vintage was f%#&ed before it started, because of the neighbors it had. Poland was dragged into modernity, because of the neighbors it had. Of course it also helped that everyone felt very guilty about their treatment of Poland in the recent past and felt they owed it one (ditto for the Romanians, Hungarians, Czechs, etc., while also recognizing that Poland benefited disproportionately from development funds in comparison to other 2004 inductees).
Some people don't feel that admittance into the EU was much of a gift, and certainly not enough compensation for past betrayals. Here I can only say, that whatever happens to be the truth: nobody likes the guy that looks a gift horse in the mouth. The transformative effect of EU funds is an established fact, and Poland has now been a member of the EU for 15 years. Aristocratically sneering at money, after 15 years of taking it, only makes you look like a hypocrite. Under the current 2014-2020 financial framework of the EU budget, Poland is slated to receive 82.5 billion euros intended for furthering the union's cohesion policy. Last year, the GDP of Poland was ~520 billion euro. So just those funds, represent more than 1/6th of the entire annual economic output of Poland. Do you still think it's just peanuts, and that only ignoramus morons speak of such things? Can you point me out a single "ore rich and oil rich" bantustan that received even 1/10th this amount of funds? The amount of EU funds transferred to Poland over the last decade-and-a-half represents a singular, and historic altruistic transfer of wealth between nation states, overshadowed only by the transfer of money from West to East Germany (can be argued it is one state and thus does not apply).