Its going to come down to all the tricks in their playbook, performance relative to their gear and others in the group filling a similar role etc. As you mentioned its hard to shine with a limited toolbox but you can still judge them against their peers for that class.
No matter what though you'll likely have a hard time shining over a good chanter/bard who is fast on the CC or a raid geared SK who looks at an overpull as a chance to self heal.
Crazy part about EQ is gear plays such a big role, especially on TLPs where some group game casual players know their shit but can't look as good as the raid geared player who knows just enough to get by and get a similar result.
For an example, I grouped with an LDoN geared warrior during GoD who had a tough time with aggroe against a group of raid geared dps. When approached with the "mash taunt" fix to his issue, he educated us on how taunt at the time was a tool to place him just above the aggroe of the highest person. By contrast we had raid geared warriors mashing taunt in a spam button and did fine because they wielded bloodfrenzy or darkblade. If that group geared tank had raid gear he'd be a stud.