Because of its high survivability, frost is far and away the most popular spec for PvP. As opposed to fire mages, who tend to die on their own, frost mages will be annoyingly resilient and tend to just not die. This is a problem. Frost mages are not concerned about burst damage, as opposed to fire mages who will dole out as much as they can before going out in a blaze of glory, or arcane mages who will pull off surprisingly painful combos with shiny missiles. Frost mages are all about control and timing. This means it's not wise to draw out a fight against frost mages, who are just as capable of producing boatloads of damage. They're still mages, after all.
Frost mages can slow opponents down like nobody's business and benefit from two effects called Chill and Freeze, which is essentially frost mage lingo for snare and root. Frostbolt and Cone of Cold both apply chill effects, and hitting mages with Ice Armor or Frost Armor on also has a chance to apply the Chilled debuff, while Improved Blizzard adds a snare component to the Blizzard AoE. Frost Nova freezes opponents in place, as does a Water Elemental's Freeze (their own version of Frost Nova). Frost also has a talent called Frostbite, which gives all chill effects a 15% chance to turn into freeze effects, and Shattered Barrier, which is a procced AoE freeze. Doesn't sound scary yet? It gets worse.
Ever heard of the Shatter combo? This revolves around the frost talent Shatter, which increases crit chance against frozen targets by 50%. With a mage's expectedly high spell crit, that practically translates to an automatic crit on frozen targets. This is why frost mages are about biding their time until they can find the perfect opening for a Shatter combo. They'll polymorph you, chill you, and freeze you just to bait your trinket so that when they catch you frozen and Shatter procs, you won't have any outs.
Here's a simple tip: kill the Water Elemental. Or, if you're a warlock, go ahead and Banish it. Water Elementals can deal a whole lot of damage from long range (45 yards) but have very little health, so ignoring them is a mistake. Mages also key bind their pet's Freeze for Shatter combos because pet abilities are on different cooldowns from their own abilities, allowing them to time their spells to abuse the Shatter proc before the freeze breaks on damage. So kill the Water Elemental.
Frost PvP is all about setting up Shatter combos up the wazoo. They won't blow a high damage spell each time Shatter procs, but they'll try to chill and freeze you each chance they get. If you play a class that can remove magic, feel free to ignore the chill effects if you want to save mana. Those will be plenty. Save those dispels for a freeze effect, because it can lead to a lot of pain. Just remember that Fingers of Frost can proc to allow mages to treat their next targets as though they were frozen. AddOns will allow you to see if they gain the buff, so you can respond appropriately with a self-dispel.
Be wary of Deep Freeze, which is a stun and a freeze. This means that frost mages can not only escape stuns every 15 seconds with Blink, they can stun opponents themselves every 30. Deep Freeze is an insurance used after cc-breaks and assures that the target will eat up the full brunt of a Shatter combo.
Even though Hypothermia will prevent it in quick succession, expect a frost mage to be able to use Ice Block twice because of Cold Snap, which allows them to reset all their frost spells. This makes them highly vulnerable for at least thirty seconds after an Ice Block, so try to deal as much damage then. An Ice Block is a full defensive move, which is a good sign to go on the offensive.
Overall, because of the proc-based nature of a lot of frost combos, it takes a bit of skill to pull off frost PvP. The good news is that a lot of players don't play at a very high level, especially in the Battlegrounds and world PvP. As long as you are able to keep off freeze effects, close the distance, and keep their pet out of play, you should be able to handle a frost mage. Melee will bother them just like any mage, and an immunity to snares, however temporary, will unsettle them.
Mages are tricky, slippery, and have a good number of tricks to control a match, but the bottom line is that they still wear cloth. Physical damage will ravage them and fast attacks will push back their casting. This is why they abhor close quarters combat. Get into close range and force them on the defensive. Casters must always be wary of Counterspell. Use AddOns such as Afflicted 3 to keep track of all their abilities -- this will help you immensely. Keep close, keep on hitting, and good luck!