You find yourself playing it for more than 50 hours.
Not so. I have played far longer than 50 hours. I would rate it as decent at best.
To me, Oblivion was a good game. Not because it was superior in every way to Skyrim or something, but the fact that I played it for well over a month, putting about 5 hours a day in, and the only reason I stopped playing is because I had done literally everything. I made every kind of character I could, completed the game three times, collected all the daedric artefacts, checked UESP wiki to make sure I hadn't missed any missions, completing missions in different ways, making a mage with the most ridiculously powerful spells or the ultimate warrior, the master of stealth and most in between. 700 hours I have put into Oblivion. 400 into Fallout 3 and around 200 into New Vegas, and I consider all of them to be very good games.
That said I consider Arkham Asylum and Deus Ex to be exceptional games too, and I have put less than 50 hours into both of them.
Skyrim, around the 40 hour mark the bugs and glitches began to exasperate me, my character was OP as hell without me even knowing about the exploits (all I knew is I wanted dragon armour, so I legit practised my smithing, without creating 1000 iron daggers). Nothing was a challenge, not even the toughest dragons, giants or draugr, I began to realise that things I was made to believe would be in the game (radiant AI, dynamic snowfall etc.) were not going to appear, the guilds were horribly short, poorly written and made no sense. Choice was more of an illusion than a presence, with literally nothing you did had any consequence or effect on the game world, not even the civil war quest completion changed anything apart from the guards appearance, there were far far too many essential NPC's which reduces the "choice" in the game, Alduin is pathetically weak, I began to miss spellmaking, and spells in general among many other issues I had with it from 40 hours onwards that don't need to be mentioned.
Once these issues (which I didn't really have with Oblivion. Sure, the story was a bit lacking and the environment was a bit bland but other than that it wasn't half bad) had cropped up, they didn't go away no matter how long I played it. I persevered with my warrior, done the main quest and then left it for a stealth character, which was good while I done the Thieves guild quests (missions were a bit lacklustre but the story was good) and the DB quests (could have been done so much better) but after that, quickly fell into the same rut as my warrior; it got boring. I've heard and seen how bad mage characters are, and I know how bad and short the mages guild story is so I'm putting off a mage and hoping that magic is seriously revamped and revised before I play one. But I've never been bored in a TES game before I have done all the quests. But in Skyrim... it just feels a bit bland. The quests all feel a bit samey, being either "kill all of these" or "find this", barring a few clear exceptions e.g. Sanguine's quest. It seemed to be style over substance, while the kill moves are nice and all, and the armour looks good, I would have much preferred to have spellmaking, seperate upper body and leg garments and better, more unique quests, less essential NPC's and more choice. Always more choice in an RPG.
Anyway, like I said that is my view, and why I don't really consider Skyrim a "good" game. Just so people who want to defend it know exactly why I am disappointed with it. (TL;DR - because it gets boring before you finish).
And to answer OP's question, I consider a good game to be something you can play from beginning to end, regardless of how long that may take, and have fun, be entertained and consider it good for the entire time.