It can generally fun, its a good grind, and some people might enjoy the feeling of fighting the enemy faction rather hunting beasts and bandits etc etc. You also can keep quest givers unavailable and gank people as you do this. Might sound evil but this is war, and after all you can still get killed by a bunch of gray lvl players, this is vanilla.
Guards can be good for grind as they give the same xp as any other NPC of their level, and also give you the enjoyment of fighting your opposed faction, plus, they are often in 1 place together, but not that close to have to aggro more than 1. The bad side is that it might be in fact not as effective for pure leveling, and its not rewarding in money.
Here are a few alliance bases to be assaulted(I will add more or you can recommend/remind me of an area)
Level 21-27 Westbrook Garrison at western elwynn forest is full of Stormwind Guards (lvl 23) like 20 of them inside the barracks and in the tower outside. You will always be busy killing them, don't take long to respawn. There also is a quest giver (noncivilian). Had a good time there.
Level 21-25 Goldshire. Just 3 guards but lots of quest NPCs and often populated by low lvls and sometimes people of literally any level. More vendors than guards to kill. (Random alliance NPCs also give xp depending on their level). Its kinda meh but why not, it has lots of civilians though.
Level 21-25 Eastern elwynn areas. There's a tower with 6 stormwind guards and a bridge with 4 guards and a strong lvl 30 quest giver, doesn't attack on sight, but don't kill him alone. You can make a route between those 2 spots, plus the graveyard is really close if you die.
Level 38-43 Astranaar in ashenvale. It has around 10 sentinel guards which are really easy to kill and they also drop silk and mageweave cloth! Also a flight path to gank, surprisingly it had honorable targets to kill too! (Contested area) can be good for grinding
Level 40-44 Menethil Harbor in wetlands, contested territory. Really populated, has many guards (aren't the easiest to kill) spread around the entire area. Watch out at the flight point, people of all levels use the ship for transportation to kalimdor. Also a good ganking spot for lvl 60s! More for PvP rather leveling.
This is for if you just feel bored to follow the normal leveling road or plain grinding, and made more for the PvP oriented. There are much more places to do similiar things but, I don't have experience of them to say details. I also wrote this cause I'm bored, but I done this my self.
Yeah xD