In TF2 hats are the rarest and most sought-after items. The vast majority of hats are purely cosmetic (there are 4 hats that are part of class sets that give you a bonus if you wear all of the items in the set). There isn't really any 'grinding' in TF2. The way the drop system works you receive an entirely random drop every once in awhile just for playing. Most of the time it's a weapon, but very very rarely it's a hat. There's a limit to how many drops you can get per week (I think it's 8, but I'm not positive) and it takes about 8 hours of playtime to hit your limit. If you don't get all of your items for the week the extras will roll over into the next week. All weapons can be found via random drops, gained through class-specific achievements, traded for or crafted. If you're trying to get a specific weapon it's extremely easy to do so.
An important thing to note is that even if you find you don't have the time to play for drops you can still get all of your drops for the week. Since the only requirement is being in-game for 8 hours a week you can simply run TF2 in offline and text only modes while you sleep one night and you'll get all of your drops for the week.
It's also important to point out that most weapons and set bonuses are 'sidegrades' to the default ones. That is, they do something better but also have a drawback. They're very much designed to foster different playstyles than they are straight upgrades. For example, the Pyro's "Backburner" flamethrower does more standard damage and always does critical (3x) damage from behind, but doesn't have the ability to airblast like the standard flamethrower. The airblast is a very versatile and useful ability that can be used to reflect projectiles back at attackers, extinguish burning teammates and push enemies away (usually to keep them from destroying sentry guns or off of cliffs to their deaths).
One other thing to keep in mind is that many of the items in TF2 were designed by players in the community and not Valve themselves. Anytime one of those community-made items sells in the store the creator gets a cut of the profits.