1) LoL gives you a rotation of 10 free champs per week, and of those, usually half are considered strong. As far as getting more, about 10 of them cost only 450 IP, which you can earn in about 3 wins, and about half of those 450IP champs are played regularly in the semi-competitive scene, with a few still popular in top-level play. And if you are willing to pay real money, they have two different collections of champs (about 20 champs each) that are only about 10 bucks. I wouldn't spend real money, but it is a pretty good deal really.
2) You obviously have not watched much hentai. They outfits are skimpy yes, about on par with 1990 Marvel/DC female superheroes. But at the same time, the characters are so small during the game, you can't really tell. And no, you can *never* spend the points you earn from playing on skins; that's the one thing you absolutely have to spend real money on.
3) The courier in DotA2 seems to me like it would ruin a lot of the strategy. A bit part of LoL is doing whatever you can to stay out in the field last-hitting minions, unless you've collected a ton of money and can by something huge. If you didn't have to leave to buy, some LoL champs would never leave lane until about 20 minutes in, giving them a huge advantage.
4) Levels don't mean everything. If you are that hung up about levels, why not play the Disgaea games, where you can level to 9999!
One reason I will never play DotA: A necessary tactic is to last-hit your own minions to prevent your opponents from getting the bonus from doing so. That's ridiculous and non-sensical.