As a new player, you'll experience this a lot. Someone will discover a really cool trick with a card, and you may find yourself asking, "Is that really what the card does? If so, then why bother buying anything else?"
The answer is that no card dominates 100% of the games. Sure, Gardens can be a really powerful card—when it works. There are plenty of times when it doesn't. The same can be said about Witch, Chapel, and all manner of cards from the expansion (seriously, go check out the BGG forums and count the threads proclaiming Pirate Ship as being overpowered).
The allure of the game (for me, anyway) is that the kingdom cards change from game to game. Part of the game is identifying when a certain card is worthwhile. Just because you got your ass handed to you by Library in one game doesn't mean that you will dominate the next game with it.
And, as has been stated, buying Estates is a surefire way to lose (except for those cases where the strategy calls for it). It's an easy trap to fall into. You win with victory points, so why wouldn't you get VP at every opportunity? In most games, this is fair, but in those games VPs are kept separate and do not hinder your ability to gain more VP later. In Dominion, that's exactly what buying VPs do. Even buying a Province is going to slow you down in over 90% of the games you play. Every time you buy a VP card, you are choosing to degrade your deck for the opportunity of increasing your score. If that card is a Province, then you have a little deck degradation for a lot of points. If that card is an Estate, then you have a little deck degradation for minimal points. Consider if you'd rather clog your deck with one Province or six Estates. Both options give you the same number of points, but the latter will hurt you a lot. For that reason, you will find yourself giving up the chance to buy VP cards early in the game. You want to build up a strong deck so that when you do start buying VP cards, they slow you down only slightly. If you can build up a deck where you buy a Province every turn (or at least a Duchy), then you're sitting pretty.