Well, Classic is well known for having very simple mechanics. There are fights from following expansions that are difficult for being a heavy DPS check, or having tough mechanics (thus hard execution), or both! Yogg-Saron 0 lights comes to mind. I guess we'll only be able to see if people can clear it quickly if and when a WotLK server comes along (or if they move Classic forward into it). But of course, there are more examples in other expansions (Like Sintharia), but Yogg 0 is one that stands out for me.