Now TF2 is free to play, and until you buy something from the Mann Co. store, you'll get some amount of nagging every time you login.
MMMPH-MPH-MMMMMPHMMPH-MANN CO. STORE! MMMMMPH-MMMMPH-MMHPMMPH-HATS! MMMPH-MPH-MMMPH-MPH-ITEMS! MMMMMHPMMPH-MMMPH-MPHMMPHMMPH-FIRE!
(There's one for every class. If you wondered why there's random class models when launching TF2 now, this is the reason.)
Now, the problem with buying from the store is you can only do it via the Steam Wallet, which can only be refurbished in multiples of $5 (5€).
Conversely, many items have 0.74/0.99/1.49/3.74 and the such prices.
How can you get premium without leaving any money in the wallet, by spending exactly a multiple of $5 (€5)?