Sethlan
The 40-man BG's (AV and IoC) are great to gear up in.
As a 5-man unit, you're only 12.5% of the entire team, which having in one place is not detrimental to your team.
Both of these are a PvE race to win, with incidental PvP going on.

If you're undergeared in the 15-man's (EotS or AB), that's 33% of the team in one place and probably not doing terribly well.
These can be won, once you have a decent level of pvp gearing.

The 10-man BG's are very hard to win as a 5-boxer; you have 50% of the team in one place always.
A 4-boxer has more success in the smaller BG's, as 4-toons can still blow people up and that lets the team be more flexible in where everyone goes.

I personally run AV almost exclusively, but will do IoC (on its weekend) if I'm gearing up, and if my team is geared will add (EotS & AB) on their weekend too.
I personally don't like SotA, nor the 10-man BG's; have never won one, as a 5-boxer.



Smalltanker
As a drastic change of pace, and in no small part due to watching Cyberus's video in The Commons, I've run my 5x Druids as Resto for the honor grind.
Initially, 1x melee would consistently take out my (quest geared, 0 Res) team.
They're almost fully geared now (have the 4.1 PvP Trinket, otherwise full 4.3 Honor gear).

As 5x casters (Resto Druids), melee aren't that much of a pain anymore.
I essentially Rejuvenate x5 whoever is attacked, and then Swiftmend x5 (for Efflorescence) and Wild Growth if necessary.
That will usually outheal 3-4 players focusing on the one toon.

It's important for me to get my 10x dots up on as many opponents as possible, and as quickly as possible.
5x Moonfire and 5x Insect Swarm does deplete their health gradually, while just healing myself eventually results in a loss.

The big cooldowns are:
- Tranquility (round-robin, frequently overlap them... starting the second when the first is half done).
- Tree Form (defensive) - I'd use this when fighting the end-boss, especially if horde run in to make it more difficult.
- Tree Form (offensive) - Wrath in half the cast time, doing 30% more damage can blow up fully geared pvp opponents in 3-4 seconds, and undergeared players fall to the second volley.

Important to note, you're running Elemental not Resto.
I'm running healer spec, and not Boomkin.
Not a perfect comparison.

My team performs very well with others around me.
And not nearly as well on my own.
Any 5x melee boxer destroys me, 100% of the time, and usually very quickly.