Mobile
+10 Speed, Dash negates difficult terrain, you can shut off opportunity attacks
Design:Again a feat that risks throwing out a bit of baby with the bathwater in its attempt at simplicity. Ignoring difficult terrain is an ability that's a tad too absolute for me, especially since not few character gain the ability to Dash as a bonus action. Still, I see what they're doing here, and I can't immediately come up with an alternative that remains simple.
Fun: Being free of restrictions is always fun. The reliability of these abilities is a good thing from the fun perspective.
Power: This is an enabler, rather than being outright powerful in its own regard.
Moderately Armored
medium armor, shields proficiency
See discussion above. This is an incredibly expensive way to gain these proficiencies compared to multiclassing (or even choosing the right race).
Mounted Combatant
advantage, safeguard mount, mount evasion
Design: First off, realize mounted combat never happens in many campaigns. This grade assumes a moderate amount of mounted combat. And I'd say this feat is right on the money for that. Safeguarding your mount and giving it evasion is exactly what you need. The mount-neutral design is good.
Fun: Advantage is never unfun.
Power: In many campaigns, this is worthless. In campaigns where you never leave horseback (griffonback?) this is a must-have. How to rate that? In-between, is how.
Observant
read lips, +5 bonus to passive perception and investigation
Design: Ouch. On one hand, incredibly worthless (other than the half-feat for wizards). On the other, potentially game-wrecking. This guy will autodetect everything, and I do mean everything since this stacks with the +5 from advantage on passive perception. The thing, however, is that I can't blame the feat for that. Getting that +5 should have worked, only it doesn't. In the end I'll have to grade the feat by the fact I hope nobody ever takes it in my campaign.
Fun: Again, if you take this feat, you do so because you enjoy lipreading. And spotting every trap. But more generally, I'll stick to a black rating.
Power: I guess black for a wizard thief. Others interested in lip reading are hosed.