Small town / countryside vibe
Feel-good, not angsty
Love triangle (ish)
Forgotten / unknown childhood connection between the leads
Similar trauma ( MLs lost their parents, raised by grandparents)
City people moving to the countryside
ML (OUST) and FL (HCCC) are medical professionals
FL (OUST) and ML (HCCC) are do-gooders loved by their community

MLs have similar jobs -- army/SWAT
FLs are doctors
Slow build-up to romance, lot of cute scenes
Good comedy that doesn't feel forced
Decent surgery scenes, accurate medical depictions, believable action sequences
Leads and supporting characters have good chemistry
The casting and acting is excellent
Good OSTs
FYI, There is very little similarity in the plot.

Both these dramas have the same premise -- a person from the present drowns and swaps their soul with someone in the past. Both these dramas have a court/palace setting, has politics and power struggles involved and has the viewer rooting for the would-be/current king.
Both the dramas have steamy romance, sweet (and sad/frustrating) misunderstandings and hilarity.
HOWEVER, Mr Queen is way more FUNNY, lighthearted and makes you laugh till you pee your pants.
Both have amazing OST apt for the scenes.
Watch Mr Queen to get over Scarlet Heart's bitter-sad ending and to get some good laughs in.

Similarities:
Both have MLs have Pposopagnosia and can only recognise the FL.
Both have a bossy guy- minion girl dynamic.
Both have MLs who are the best at what they do.
Both have MLs who are rich and FLs who are from a poor background and have to take care of their younger siblings.
Both have MLs taking the help of FLs to help interact with other people.
Both have FLs pretending to be someone they're not.
Both have MLs who have poor relationship with their mothers.

Differences
One is a college romance, other is an office romance.
One is a KDrama, other is a CDrama.
The FL pretends to be a boy in 'the unicorn girl' whereas in the other one, FL pretends to be rich.

The Prince of Tennis has no romantic story-line. If you're hooked on the passion, fervour and spirit of working hard and chasing ones dreams, you get more of that (X10) in PoT.

Plus, the guys are all hot!! Beyond this superficial reason, it has similar themes of giving it your all, pushing yourself to better yourself, being passionate about your goals and most importantly, never giving up.

The female leads are sassy and adorably dense. They're damn supportive of their partners. The male leads are swoony in the way they care for their girl.
Both are sports dramas.
Both have a theme of pursuing dreams and love.
Both have sweet relationships.