You can continue in several of the zones I listed for the 30′s until level 45. Here I’m going to cover the ones for you in the 40′s.
Azshara (48-55) -In Kalimdor named for Queen Azshara of the Naga which still haunt this area. This land is considered cursed by the Night Elves. It was destroyed during the Great Sundering. This zone has a reputation of being as far away from the rest of the game as possible. If you’re bad about falling down cliffs, like I am, then it can be a long walk a long way around to get back. There is an alliance camp near the border of Ashenvale & Azshara with a flight point.
Blasted Lands (45-50) – In the Eastern Kingdoms, this barren desert used to be part of the Swamp of Sorrows until the Dark Portal was opened. For Alliance, Netherguarde Keep is here. You will be returning to the Blasted Lands to go through the portal to Outlands.
Felwood (48-55)- In Kalimdor, this is a corrupted forest inhabited by Satyrs. Also home to one of the pure tribes of furbolgs, the Timbermaw. This is a faction that you start off as Unfriendly with because according to lore they are distrusting of strangers. If you quest in this area I recommend you try to keep doing the repeatable turn in quests until your reputation with them is at least Neutral. This will allow you through the Timbermaw tunnels to reach Winterspring & Moonglade. Alliance has a camp at Talonbranch Glade in northern Felwood with a flight point. Horde camp is at Bloodvenom Post in middle Felwood with a flight point there. Cenarion Circle (a neutral druid faction) has a camp at Emerald Sanctuary with a neutral flight point there.
Feralas (40-50) – In Kalimdor, this area is a lush forest. The ogres inhabiting the Dire Maul ruins & surrounding area have tainted the area. The Grimtotem Tauren tribe have a presence in the area that I believe are hostile to both Alliance & Horde. Feathermoon Stronghold on the island off the western coast is the Alliance base in the area. Generally it is recommended to wait for the boat instead of swimming because of mobs in the water. Alliance also has another flight point on the eastern end of the zone at Thalanaar, but be careful of just following the road if you are Alliance. It leads straight through Camp Mojache. Horde will have a flight point at Camp Mojache. Dire Maul contains a 3 wing dungeon for upper 50′s level.
Searing Gorge (43-50) – In Eastern Kingdoms, a desert area with quarries dominated by Dark Iron dwarves. Thorium point is a neutral camp run by the Thorium Brotherhood & flight points can be found there. Entrance to Searing Gorge can be a bit tricky. For Alliance the gate from Loch Modan is locked until you obtain the Key to Searing Gorge (link) via quest line. A rogue of 225 or higher lock picking can also open the door. You can also enter by traveling through Blackrock Mountain or from entering via the path near Kargath in the Badlands.
The Hinterlands (45-50) – In the Eastern Kingdoms, the Wildhammer dwarf clan, a shamanistic believing clan of dwarves and are famous for their trained Gryphons, make their home here. They are quite loyal to the Alliance. Several clans of Trolls still remain in this area, but only Revantusk are loyal to the Horde. Revantusk Village hosts the Horde flight point & Aeire Peak hosts the Alliance flight point. As both Horde & Alliance you may enter this zone from Hillsbrad Foothills.
Un’Goro Crater (48-55) – In Kalimdor, Un’Goro is a huge basin jungle.dinosaurs roam the landscape. There is a volcano in the center. Most WoW players will swear the the giant devilsaures have learned to tip toe, & will sneak up on you when you least expect it. Neutral quest hub & flight point can be found at Marshall’s Refuge in the northern end of the zone.
Maraudon (45-48) – Located in Desolace, this is a two sided dungeon. You will frequently hear references to the purple side & the orange side. Lots of Centaurs. If you are a caster or healer beware that mobs frequently silence you here.
Sunken Temple(47-50) – Located in the Swamp of Sorrows, this dungeon took me multiple trips with a guide article to figure out. It is also known as Temple of Atal’Hakkar. It’s filled with Trolls & member of the green dragonflight.
Warsong Gulch & Arathi Basin as previously noted