*sip* Mmm... Thistle Tea. Sounds tasty, doesn't it? I think it tastes like swamp, but regardless, nothing gets you moving like a good cup of Thistle Tea. *sip*
While the talents and gear to raise your maximum energy aren't available at lower levels, Thistle Tea is a drink that when consumed gives you an instant burst of energy. Thistle Tea can be consumed as early as level 5. While it might seem like a young age at which to begin experiencing the joys of mind-enhancing drugs, there is no Alliance or Horde Drug Enforcement Agency to police it. And even if you aren't able to craft it or can't collect the ingredients yourself, you can still use it.
As great as Thistle Tea is, it does have a 4-minute cooldown, meaning you'll have to do some planning ahead to make optimal use of it. Also note that, even at the earliest levels (its potency unfortunately diminishes as you gain levels), the maximum energy granted by Thistle Tea is 100, the same amount of energy recovered by a Rogue over 10 seconds.
Thistle Tea is a rogue-only Cooking recipe. It is available as a quest reward from the level 24 quest, Klaven's Tower (Mission: Possible But Not Probable if you are Horde). A Cooking skill of 60 is required to craft it. So, grab a fishing pole and don an apron as fishing in your hometown and cooking the fish you catch is one of the quickest ways to raise your Cooking skill to 60. Now that you can cook Mac and Cheese without burning down your house, you're ready to brew your first kettle of Thistle Tea. To do so, you'll need 1 Refreshing Spring Water and 1 Swiftthistle. The water can be purchased from vendors, but the Swiftthistle is an herb rarely spawned when gathering Briarthorn and Mageroyal. These will be harder to come by and are one of the reasons Rogues should pick up Herbalism, especially at early levels.
Despite not being able to learn the recipe until at least level 16, from level 5 to level 39, your Rogue should become a Thistle Tea junkie. Find yourself some from the AH or bribe an older rogue to make it for you. Drink it whenever your Energy dips near zero. Why?
Math time. One Rogue leaves Darnassus and another Rogue using Sprint leaves Stormwind... wait, wrong page... Say it takes you a minute and a half to kill some monster. During the course of the fight, you'll regain 900 energy (90 seconds * 10 energy/sec). While you can't change the rate at which you naturally regain energy, you can down a Thistle Tea for an extra 100 energy. At best I'd guess you can utilize between 60-90 of the energy due to timing, but that's still a 6-10% increase in your available energy. Even at 60 energy, that's a Backstab or almost two Sinister Strikes. During a longer fight, using it every four minutes, you're looking at an extra 2-4% energy. Energy equals damage. You want to do more damage, right?
At level 40, you start getting less energy for each consumption of Thistle Tea. I like to attribute this to a Rogue's growing tolerance for the bitter drink, rather than Blizzard's nerfing of a class staple. For every level starting at 40, you get 2 less energy per level as you drink it until you only get 40 energy back at level 70.
This reduction in benefit changes things significantly for the conscientious consumer of consumables. At level 70, you only get 40 energy. For a raiding Rogue, that's 40 extra energy every four minutes which is a boost of 1.6%. Thistle Tea goes from a substantial boost at earlier levels to a nice addition during raids. I'm sure it's because of the introduction of the talent Combat Potency which gives Rogues a new source of energy recovery starting at level 45. It is also probably to reduce Rogue's dependence on an item that can be difficult to acquire for higher end characters.
There has been a large outcry from the Rogue community for something to replace Thistle Tea in the Outlands, even if it recovers not more energy but requires a new herb that is more accessible to players in the Outlands. So far, the East Azerothian Trading Company hasn't seen fit to reduce the tax on regained energy or introduce a new recipe as an alternative to this class addiction.
Tea party, anyone?