They are allowing mythic eggs to be bought and traded, most gear can be bought if you grind and get good drops especially on elite maps. In fact this time Slag cant be directly bought so I'd say its an advantage for free players.
Nuked lower level players can be higher level in around a week of grinding. Sure they can't enjoy every event to its fullest due to the limitations of their level (like killing Tarlok at lvl 15) but that is to be expected. Their gear is not expensive to obtain and reaching lvl 25 is fairly simple. At this level most really good gear can be purchased for less than 10k. Also this is an add on event that makes it interesting. First the holloween and now this. You can't really say anything that justifies this being a pro pay game.
However some amount of payment for gear is also justified unless you like to work for free. This game is very balanced as far as pay vs non pay goes and I'm hard pressed to find fault with the developers. If anyone puts in the time they can get all the gear from mythics to arcane
Bookmarks