I don't mind micro-transactions and in some cases they work quite good. Obviously, as a gamer I always dislike it, but I respect it from the business side, and it's quite common in Asia. Most every app or game costs money to play. Cards game on QQ or with Tencent, puzzle games like Candy Crush, or basically every Asian RPG in existence (I know a few wealthy 'second generation rich' that have spent over $100,000 USD on MMO items, I shit you not). It's very normal. Almost every guy I went to business university with from China spent at least thousands on game microtransactions (but they're all relatively wealthy by North American standards, so it's not a directly fair comparison to average US consumer). But this is acceptable behavior by most every gamer or even non-gamer in age 20-35 age group, really. It's just how apps and games work there. Point is, this type of game design is increasingly becoming a normal way to market content.
The key difference is almost every single example is 100% free to only download but even play under certain limitations. This is where the Forza 5 situation feels a little tasteless because it's effectively charging you a far higher price than $60 USD for a launch game and its full pre-launch content.
Though I do have a somewhat unrelated question about the slow grind in Forza: is it the case that the $-per-hour average quoted may be inaccurate in the way that if you took an average in Gran Turismo over the first few days, it would likewise take forever to buy nice cars, yet we all know that later in the game the cash is earned at a far faster rate -- this would result in a less grind-y game and make paying for the DLC much more optional and not forced. As long as people can reasonably earn these cars as they would expect in a previous Forza or GT, and its not blatantly discouraging this and encouraging payment of content, then maybe it's not so tasteless. But again, this could be very dependent on how fast you can grind later in F5 and if that average is genuinely accurate over the lifetime of the game. Or, of course, if any notable portion of that content cannot be earned outside of micro-transactions (just in case I misunderstand because I haven't followed Forza).