Potency in what they say, vocabulary, word usage and effectiveness, rhymes. Those are the most important aspects to me, then other techniques such as wordplay, punchlines etc, I like when an MC can be witty with their lyrics.
Add poetic to this also, someone like Common has some nice ish in that regard, Nas, Brother Ali, Atmosphere.
Unfortunately a lot of people on here don't really know what lyrical is, being simple as possible isn't synonymous with being lyrical, infact that makes it less lyrical. Someone like Biggie was lyrical though because if you look at the potency, rhymes and other aspects you can see he was really a dope lyricist, Big would drop internal rhymes all the time, he had an unorthodox rhyme style that was exceptional, made it work though cause he was blessed with a great voice and was a phenomenal MC.
Pac another artist who was a great lyricist, potency in what he said was exceptional, often had a great set up too and structure, that seems simple but takes skill, if you don't build the rhyme properly properly where it needs to be the potent part where you're trying to really emphasise a line/message won't work. You can have a style like Pac or a style like Chino XL and still be lyrical, and make dope music too. There's different ways to be lyrical....