Patch 5.1 introduced pets to the vanilla-era raids. Patch 4.3 introduced transmogrification, which brought old raids back. Before that, some players would solo or duo old raids just for the challenge giggles. Every new expansion has raised the level cap and opened another expansion to soloing.
Whether you're soloing for a look, for improved familiarity with your spell toolbox, or for bragging rights, warlock is definitely a great class to jump into soloing old raids. At level 90, even before you're decked out in tier 14, most of the Wrath of the Lich King raids are fairly easy. But let's start with the basics -- speccing and playstyle.