So being in the first PHB, like the Cleric and Bard, meant the Warlord got a lot of support, with several different builds emerging over the course of the edition. While not the healing powerhouse of the Cleric, the Warlord had access to a Feat that let them use their minor action encounter healing power an additional time, which was very welcome.The Warlord had, from 1st level, abilities that let them give up their mediocre Leader attack to grant a Striker a bonus attack, which was not only an upgrade in of itself, but also allowed for the "Lazy Warlord" build, that never actually attacked themselves, but lent attacks to their Striker allies.
The Warlord also had powerful abilities that could turn the tide in battle, such as a Daily area heal in Stand the Fallen, or utility powers like Reorient the Axis, which allowed all your allies to move freely to better flank foes (or just get out of danger).
One thing you can't underestimate is the Warlord's Action Point ability; any time an ally spend an Action Point, they got a non-trivial buff. Probably the best of these was the Tactical Warlord's ability to grant a bonus to all attack rolls equal to their Int bonus. In a game where attack bonuses can be rather stingy, adding a +4 or higher to all your attack rolls during a nova round, when you're firing off Daily attack powers, is a godsend for just about any character. The Eladrin, in particular, had a Paragon Path that increased accuracy to near game-breaking levels; don't forget that many powers that have saving throws now instead targeted a non-AC defense. This means that landing powerful "until end of next turn" powers on Elites and Solos is incredibly satisfying, and increasing the odds of success by a large margin is incredibly good.
Higher-level abilities allowed the Warlord to give their entire party free attacks, like charging and allowing all your Strikers to also charge off turn. There was even a decent ranged Warlord, with powers like Race the Arrow, which allowed you to shoot an enemy, and then let one of your allies charge that target for free with bonuses.
The biggest draw, however, of the Warlord, I think, was that they provided these benefits through tactical acumen and leadership, not divine inspiration or magic. For players who are averse to "finger wiggling", this was the perfect complement to a martial playstyle.
I'm actually surprised you saw so many Ardents, to be honest. Maybe it was their choice of build and powers, but the one guy I played with who had an Ardent always seemed to come up short compared to other Leaders; even my Bard regularly showed him up.
As a side benefit of the "Lazy-lord" build, the Warlord was incredibly useful for Hybrid builds, granting strong armor to classes that lacked it, and always-useful support abilities. Probably my toughest character in 4e was "The Professor", a Hybrid Wizard/Warlock who could wear medium armor and a shield, and who generally had an answer to any situation his group got themselves into, even if that was "throw the Ranger at it".
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