In my mind, a "fair" microtransaction (or even not so micro) is one that does not offer a competitive advantage unless a similar advantage could of been gotten with a reasonable amount of time, effort, and/or skill otherwise. So basically, paying can give a boost, but it must be something you could get otherwise (but maybe take longer or be harder), or be not all that great in competitive impact.
For example, paying to get a boost in experiance? Fine, You could get that experiance anyways through enough play and skill.
Paying for a new costume that does not alter stats (or at least not significantly)? Fine, as that is not a competitive advantage.
Paying to get a sword that has +30 to HP and +5 to attack but you must be level 5 or up to equip, but there is another sword that has around the same boosts but can only be equipped at level 15 or up? Well, I would shake my head at that, but mostly fine, as although that does mess with competitive balance in the earlier stages of the game, in the upper "tiers", both pay-for and not-pay-for can get similar levels of rewards.
Paying to get a sword with great stats, or getting a absurd amount of in game "money" to buy it (or something similar) instead? Again, I would shake my head at that, as it is bordering on violating the "reasonableness" stipulation, I would probably be fine with it, so long as the "conversion rate" isn't absurd as well. Like, a dollar would give you about a weeks worth of grinding amount of in game currancy/items, yea that would be reasonable. A dollar would give you about a years worth of grinding of in game currancy/items, no, not reasonable or fair.
In my mind, a "pay-to-win" game is a game that is dominated by not fair transactions as given above.