The 70-79 bracket

Northrend is quite an interesting place.  The gear disparity between a player just exiting Outland and one about to hit the level cap is incredible.  Blizzard just throws gear at you.  In the ten levels between 68 and 78, I've nearly doubled my stamina and intellect, tripled my spell power, and gained a decent amount of hit, crit, and haste, which were before quite low, if even existent.

This is very fun, and an effective way to motivate players through the drudging 1.5 million experience points required to make it past each level.

There are a few problems caused by this, however.  Everyone knows you shouldn't play in battlegrounds if you are in the bottom of a bracket, but this is even truer in the 70-79 bracket.  In lower brackets from Classic WoW, there might be a few attribute points of difference between non-twinking players, but nothing major.  At that level in the game, significant factors come from class abilities and hit chance.  At the WotLK level, the differences between gear are huge.  Every other quest throws new gear at you, and because it takes so many quests to level up, gear disparity between yourself and a player just one level above might be as great as what you would have seen before when compared to a player ten levels above or more.

But if that didn't already complicate things for PvPers who don't want to wait to level to join battlegrounds, there are other things to consider that make the 70-79 bracket difficult for anyone not very close to 80.

Level 78 PvP sets
On my first Boomkin, I had farmed leather for several days with my leatherworking friend and guildmate, so that he could make me my Overcast set.  Granted, this set is better suited for Restoration Druids, but this wasn't such a problem.  The spirit to spellpower from Improved Moonkin Form compensated fairly well for the total lack of critical strike rating.  In fact, it gave me an additional 90 spell power, which is a nice amount at that level.

Level 78 PvP sets require a lot of materials, but somehow everyone has them anyways.  Several of my friends had them, and from what I could tell, so did a substantial amount of other players.  They're great as an introduction to PvPing at level 80.  I imagine Blizzard introduced them to compensate for the even greater gear disparity you can find at the level cap.

But in essence, these sets are ridiculous.  I had all 8 pieces of it ready and waiting when I hit 78, and equipped them all at once.  I suddenly found myself with 4k additional health, a considerable spell power boost, and went from 0 to 300 resilience.  Excited, I joined a Warsong Gulch and I was amazed at the difference it made.  Suddenly, I felt like a walking tank.  I hit hard and it was more difficult to take me down than ever before.

Farmers
But if having to fight against full blue PvP-geared 78+ players wasn't bad enough for lower level players in that bracket, as it turns out you often won't find anyone in the bracket that isn't level 78+.

A lot of people at that level have the notion that farming honor at 80 is insanely hard due to epically geared veteran 80s, and choose to stay at 79 for a while to build up honor.  Not to mention that players generally join in battlegrounds when they are among the higher levels of that bracket.

Conclusion
If you just dinged 70 and you're so PvP-hungry that you don't want to wait until you've leveled up a few times to start doing battlegrounds again, here's a few tips.

1. Be discreet.  Don't get yourself noticed, and you might be able to provide good help to the team.  That means you probably shouldn't try to carry the flag yourself.  The main issue with being a lower level is that you are very easily killed, so try to get yourself focus-fired the least amount possible.
2. Stay close to someone.  If a high level player singles you out, you're done for.  Stay close to someone who can help you if needed, and who you might also be able to help.
3. Play a healer.  At first, this may seem contradictory, because ideally, the opposite team should always be focus-firing either your flag carrier or your healer.  But that said, we don't live in an ideal world, and most players - especially not before 80 - don't know how to play smart, and you can usually get away with healing without having the entire opposite team suddenly concentrate on you.  Furthermore, you're a healer.  If you play the role well, you can survive decently well under fire, even if you're lower level.  That said, you should still abide by the first tip - be discreet.  If you're a Resto Druid, you might want to stay out of Tree Form, unless you're already being meleed.