Edit: The following is falsely interpreted and should be largely ignored unless you want to laugh at my folly.
I'm a big fan of the paladin's Find Steed spell. At first glance it seems extremely weak, but with a bit of work it can be extremely powerful. It's a second level spell, so paladins get it at level 5. Keep that in mind. All of this is from a level 5 paladin.
"You summon a spirit that assumes the form of an unusualy intelligent, strong, and loyal steed, creating a long-lasting bond with it. Appearing in an unoccupied space within range, the steed takes on a form that you choose, such as a warhorse, a pony, a camel, an elk, or a mastiff. (Your DM might allow other animals to be summoned as steeds.) The steed has the statistics of the chosen form, though is a celestial, fey, or fiend (your choice) instead of its normal type."
That bolded section there gives you an incredible amount of freedom. The rest of the setence lists examples (that you don't necessarily have to pick from), and then says the DM gets the final say. After a short conversation with my DM, I got him to allow me to pick the form as long as I could justify it a decent amount.
My gold dragonborn, paladin of Bahamut (the god of metallic dragons) decided to go with a young gold dragon. The text says, I explained, that I summon a celestial that takes a form I choose. Surely a celestial sent from the realm of the dragon god would be capable of taking the form of a dragon. That alone gave my PC a massive boost in power.
The spell text continues: "Your steed serves you as a mount, both in combat and out, and you have an instinctive bond with it that allows you to fight as a seamless unit. While mounted on your steed, you can make any spell that targets only you also target your steed.
When the steed drops to 0 hit points, it disappears leaving behind no physical form. You can also dismiss your steed at any time as an action, causing it to disappear. In either case, recasting this spell again summons the same steed, restored to its hit point maximum.
While your steed is within 1 mile of you, you can communicate with it telepathically."
Those two bolded parts there make it seem like the summoned steed acts as one with you. What this means is undefined (to the best of my knowledge), so I take it to mean the player controls both steed and PC on the same initiative, sharing a move action but both getting their own standard and bonus actions. So my young gold dragon can swoop down 40 feet, breathe fire on the enemies, paladin takes a couple swings, then both fly 40 feet away again. Give the rider a reach weapon (I use a glaive) and you can fly high enough to avoid opportunity attacks.
Overpowered as that is, it gets ten times worse with the Mounted Combatant feat. Not only do you now get advantage on your attacks against most enemies, but you can redirect all attacks against your mount to you. Paladin with 18 AC from plate, +2 from a shield, apply the shield of faith spell to yourself for another +2 AC, putting you up at 24 AC against attacks. Now keep in mind that you're flying so only ranged attacks will be going for you, and your mount provides partial cover for an extra +2 AC. And to top it all off, you're preventing your mount getting hit so you don't have to worry about potential fall damage.
But, I hear you say, that's only because your DM gave you a dragon. Yes, that's true. Let's say I went for a Griffon instead. The flavour text in the MM even mentions that they "can be trained to serve as a mount." (Page 174, if you were wondering) Looking at the stats, you lose its reach attacks, so it'll have to fly lower to fight. That means opportunity attacks, but that's fine. You can still redirect attacks away from it to yourself. It has two attacks, plus you still get two of your own, meaning four attacks with hit-and-run tactics so enemy melee can't get their own attacks (excluding opportunity attacks) off at you. With fly 80 you will fly just as high as with the dragon, and you'll get the same partial cover against ranged attacks, so no worries there. It has darkvision, and has its mind linked to you, so you effectively gain darkvision through its eyes in case you didn't have it.
What's that? Griffon is listed as a monstrosity so it's not allowed? Ignoring that the only mention of animals is that your "DM might allow other animals", and that it says nothing about not allowing other types, and that "the steed takes on a form that you choose", I'll play. Let's take the warhorse. Now this warhorse is a special warhorse. Keep in mind that we "fight as a seamless unit" and that I can redirect attacks against it because of my feat. Let's use its speed 60 to charge an enemy thirty feet away. Warhorse hits, target has to make strength saving throw or be knocked prone. If it fails the DC 14 and go prone I'll take two attacks (with advantage) at it thank you very much, before using the rest of the speed 30 to ride away. Even if knocking it prone fails the feat still grants advantage on my attakcs so long as it's not mounted on something as large as my horse. If its still alive, my foe will take an opportunity attack at my steed. Wait a sec, I'll make it target me (still with my 24 AC). And it has disadvantage if it's prone. I'm not too worried about the chances there.
I'm sure you're all thinking by now that I'm a horrible person to do that to my DM. In truth, I agree with you. I was a jerk for breaking it that hard, but there are a couple weaknesses with it.
1. This is the big one. Ultimately the DM can just say no to everything. The same rule applies to any spell (or anything else in the game for that matter), but it is something to keep in mind.
2. Spell casters. There is a reason I didn't mention them. If you only get the warhorse then there are several rather weak saves that could be targeted. No proficiencies, no modifiers higher that +1 except for strength. Ouch.
3. Area attacks. I can redirect targeted attacks to myself, but I can't do much about that cloudkill except try to outrun it. The steed still only has as much life as a regular mount, so it'll drop very easily at higher levels.
4. The DM, part two. The spell makes the mount intelligent to the point that it knowns a language. Its intelligence increases to 6 if it would otherwise be lower. Nowhere does it say that the mount is your servant. The DM could simply say "your steed understands your command, but thinks it is too risky and refuses."
5. If you're not outside that day then none of this applies. When was the last time the BBEG in your campaign has his lair open to the sky?
My DM and I realized just how absurdly powerful this spell could be after the incident with the dragon, and we decided to come up with some house rules to prevent it in the future.
1. The paladin has to have a good* reason for why they want to summon that particular form.
2. The paladin must roll d20 + spell casting modifier + proficiency bonus vs 9 + that form's CR + that form's proficiency bonus. For the young gold dragon that works out to 9+10+4 = 23. Most paladins won't be rolling that reliably until later in the game. On a failed roll the spell fails and the slot is expended.
*good is at the DM's (or table's) discretion
In short (well, not really short at all), that is why I find the Find Steed spell ridiculously powerful.