The rules as written don’t allow for the possibility, the term “grease” covers a variety of substances, some of which don’t burn easily, and then regardless there is every reason to think that magically-created grease is not literally the same substance as any mundane grease and therefore would not share the properties not listed in the grease spell description.
On top of that, there is strong circumstantial evidence: in Mythic Adventures, there is an augmentation of the mythic version of grease that makes it flammable. 3.5 had a similar case, a 2nd-level incendiary slime spell that was “grease, but flammable.” These things wouldn’t exist if grease itself were already flammable.
Moreover, grease is one of the best spells for its level in the game, which makes many DMs leery of opening up yet more features for it. On the other hand, DMs also like to reward creativity, so some do allow grease to be lit on fire. You will have to ask your DM for a more solid answer than “the rules don’t say you can, so you can’t.” Or, if you're the DM, you should follow the line of reasoning you prefer to its conclusion.
My first character ever, a sorcerer, had grease as something of a signature spell, and this was one of the first “tricks” I tried with it. The DM didn’t allow it, and considering how much use I got out of the spell despite lacking that option, I think he made the right call, and I’ve made the same call in all of my games since. I have never seen anyone complain that grease is unusable without that feature. Again, it’s one of the best spells out there.
The Sword of Sharpness has a utility as a magic item that is somewhat variable in that it depends on the player who has it, and on the DM. We'll take the features one at a time.
Light Source
This is fluff. Moving on.
Maximized damage against objects
The utility of this depends on the player and how creative they are in using it. If you never attack an object, this is pretty much useless. However, you can get a LOT of mileage out of attacking objects. Personally speaking, I've busted my way through walls to circumvent an ambush or trap. Cut the hinges off a door so I could remove it without destroying it (the rogue broke the lock while trying to pick it). I've cut ropes to drop chandeliers on my enemies, busted holes in an enemy's boat so we could escape while they sank, and wrecked the DM's death traps by destroying the mechanisms that made it work (he wanted us to go through this complicated disarming process...I worked all the delicate looking moving parts over with a Maul. It worked). Those scenes that turn up where, say...Wolverine carves his way through a wall with his claws? With a Sword of Sharpness, you can do that.
The ability to reliably deal maximum damage to objects, and reliably overcome most objects' damage thresholds if they have one, is really useful in the hands of a creative enough player.
Ask your DM about the environment, look around, think about what you can do with what you can destroy. That door doesn't have to be the only entrance...those light fixtures can be weapons.
Again, this is all stuff you can do with an ordinary weapon...but a Sword of Sharpness is much more likely to finish the job in fewer attempts.
Bonus Damage
Boosted damage on a Nat 20 is nice. Barbarians, for example, are partially built around their boosted crit damaged. And, per the DMG, it's a flat +14 damage on a nat 20, which has more reliability than 4d6, even if it has lesser peak damage output. To address your comparison to the Flametongue...bear in mind that the Flametongue's bonus damage is Fire Damage: one of the most resisted damage types in the game.
Limb Removal
Here is where we hit DM caveat territory. What does it mean to dismember a target? Well, per the DMG, that is going to depend on your DM, and it's going to depend on what you're attacking. If you take a limb off a Zombie or a Troll...no big deal. If you take a limb off something that is living and doesn't regenerate at an insane pace...they are in trouble. Let's walk through what happens here, realistically.
- You lose the use of that limb. Depending on if this was a leg or arm, it has a different impact, but it's simple enough to figure out.
- Here's the important bit. You immediately begin hemorrhaging blood rapidly. Taking an arm off severs the brachial artery, taking a leg off severs the femoral artery. With the degree of blood loss this would cause, you will be dizzy and drowsy almost instantly, unconscious within seconds, dead within a minute or two. They'll last longer if it is a 'clean' amputation (perpendicular to the line of the limb) because arteries can pinch themselves shut...but that doesn't work if the cut isn't straight across, and since this happened in combat, you probably didn't give them a nice perpendicular amputation. And even if you did...that level of damage and pain is going to put just about anything into shock...which, however the DM portrays that, certainly puts them out of the fight entirely.
Now, is your DM using it this way? That's DM caveat. But in a game that I am running, limb loss = immediate removal from combat and massive ongoing damage. Unless medical care is administered immediately, the victim of that attack is going to die. Ultimately, in one of my games, the difference between a Sword of Sharpness taking a limb, and a Vorpal Sword taking your head is whether or not you have a chance to save the victim before they bleed out.
Summary
If you don't come up with creative uses for your newfound ability to obliterate inanimate objects and your DM treats limb loss like an inconvenience, then I would agree with you that the Sword of Sharpness probably doesn't deserve its Very Rare rating. But if you get creative, and your DM treats limb loss like the catastrophic injury that it actually is...then it rates quite a lot better. And...+14 damage on a nat 20 is pretty significant (equivalent to average damage of a 13th level Barbarian's Crit damage bonus with a Greataxe), and since it is Magical Slashing damage, very few things are going to resist that.
I would personally rate this on the lower end of the Very Rare magic items...but I still think it deserves the title
Best Answer
Not by RAW: "bright light" =/= "sunlight" =/= "The Sun"
None of the rules indicate heat as a characteristic of the various light spells, although some "are sunlight" and can do radiant damage. The light cantrip is not described as being sunlight, nor "as bright as sunlight." Light emitting spells that might suffice:
Sunburst
That spell doesn't last 5 minutes.
Daylight.
It lasts long enough, but isn't sunlight.
Light
Lasts long enough, but it isn't sunlight.
Dawn
Yes sunlight, not a 5 minute duration.
Note: Whether or not radiant damage includes "heat" or is some other kind of magical / holy radiation is unclear. Not all light sources can do what the sun does.
Is this a cool idea for lighting a fire? Yes.
Under Rules as Fun, you could rule as a DM that with a bit of time and effort, a cleric could (with a suitable magnifying glass, or with a similar item like a lens from a spyglass (Expensive!)) concentrate enough heat from the light into something that starts a fire. But a tinderbox is a lot cheaper. Do you want to dismantle that spyglass, and perhaps not be able to use it?
There are other cantrips that can light a fire as part of their basic function: druidcraft, produce flame, or that old standby prestidigitation