My druid was originally supposed to be my tanking alt. Back when I created her, I already had a healer and a dps character at the level cap on Earthen Ring, so levelling a tank as my third toon seemed like a logical next step. This was back in BC and it worked out well at the time, as I really enjoyed being a feral tank. Towards the end of BC my boyfriend encouraged me to level my paladin alongside one of his alts, which I did, but with no particular purpose for the character. She was retribution most of the way for ease of killing things, and I only respecced her to prot when I wanted to do a normal Nexus run with friends one day and we just couldn't find a tank (this was pre-dungeon finder, obviously).
And yet, over the course of WOTLK my paladin somehow ended up becoming my main tanking character and my druid is resto most of the time. I still work on her tanking gear, but I only really put it on as a favour for friends and guildies; on random runs I'm staunchly resto. As far as I'm concerned, druid tanking is just too lacking in several areas compared to paladin tanking these days.
1. Caster mobs
Once upon a time, every tank worth their salt had to learn how to make a corner pull at some point in their career, because it was the only way to force caster mobs to come into melee range along with their mana-less brethren. It was sort of an art form, if slightly annoying at times, but I didn't mind having to do it because it was simply part of being a tank.
Then WOTLK came around, death knights appeared with their shiny death grip, avenger's shield was buffed with a silencing effect, and protection warriors got heroic throw combined with gag order, so they were all able to maneuver at least one caster around as desired all of a sudden. Druids however were still stuck having to make line of sight pulls, in a world where every other tanking class quickly got the dps used to the idea that strategies like that were obsolete now. Yeah, good luck with that!
I'm not really someone to promote complete homogenisation of all classes, but this one just irked me, and continues to irk me every time I have to tank a pull with casters on my druid and find myself thinking "damn, this would be so much less annoying if I was on my paladin". It's quite an important part of tanking, and I still don't get why Blizzard made a point of giving every tanking class but druids a helpful tool to deal with casters in WOTLK. Why do bears have to play with a handicap in that regard?
2. Boring AoE rotation
AoE tanking is by its very definition not as interesting as having to focus on one or more targets individually, but on my paladin it doesn't feel that bad because I still get to use a variety of different skills. Yes, I'll lay down a consecrate if there are a lot of mobs, but I can also hit three (or four if you've got the glyph) targets with hammer of the righteous, then slap a judgement on the mob with the highest health and follow it up with a shield of righteousness... the fact that none of my tanking abilities are spammable and I have to alternate between different spells makes things interesting.
On my druid? Swipe, swipe, swipe, and nothing else. If I have enough rage I'll also keep mauling with every auto attack but that's it. Trying to do anything else, like putting a lacerate on every target, will simply result in insufficient threat generation in this day and age of AoE damage. Unfortunately doing nothing but spam swipe and maul gets old really quickly.
3. Wonky single target threat
On the other hand, building threat on a single target like a raid boss is fiddly and extremely unforgiving as a feral druid. Mangle and maul work well for initial aggro, but to really keep your threat up you also have to build up and maintain a stack of five lacerates - and this while a single lacerate application doesn't actually produce that much threat anymore. This is bad because if your stack falls off for whatever reason - be it you hitting the wrong ability while trying to move out of the fire or the boss undergoing a phase change or whatever, you have to start over and accept that you'll produce diddly squat threat for the next ten seconds. I absolutely hated bear-tanking the two jormungars in Trial of the Crusader for that reason for example - after the first burrow I'd just spend the rest of the fight taunting constantly because I was always behind on aggro.
Now compare that to my paladin, whose threat generation is through the roof no matter what I do. I'm vaguely aware that there's some kind of rotation (6-9-6?) that you should be following, but I've never even bothered to read up on it because I simply didn't have to. I just use whatever threat ability isn't on cooldown and for the large majority of groups that's more than sufficient. Sure, it's not perfect and every now and then there'll be a gap in my rotation (which is usually where I improvise with an avenger's shield or something), but it's not a matter of life and the warlock's death.
Is it true?
Now, this is just my personal assessment, and other people might still prefer the bear style of life over paladin-tanking (though I don't know why). I still occasionally hear about players struggling with the pally's mana bar, but personally my tankadin's blue bar never goes below fifty percent or so since they made it possible to keep divine plea rolling non-stop.
Just from my own observations in random heroics, paladin tanks seem far more common than any other type these days, with druid being one of the, if not the rarest. They might still be competitive in a raiding environment, but as far as the pure fun of tanking goes, I think bears have lost a lot of appeal.