1. AFAIK, most standard acoustic guitars have scale lengths similar to electrics.
 
2. With a few exceptions, the unwound strings in acoustic string sets are identical plain steel strings as used in electric sets. The gauge of course matters, and as discussed above, popular acoustic sets are often heavier gauge.
 
3. String tension is a function of pitch, scale length and the mass of the string, which is determined by the string's gauge, material and construction (wound vs. unwound, etc.).
 
4. String length outside the speaking length of the string (i.e. the length outside of the nut and bridge saddles) can affect the feel of a bend while bending, but ultimately reaching a given pitch is based on the factors listed in #3 above.
 
Pretty much everything else is on the player in terms of proper technique (!), hand strength, etc.

karma1959
- String gauge (obviously heavier gauge strings are harder to bend)
- Pitch - as above, some players tune down a half step to 430, which obviously makes things looser than 440.
- Guitar's scale length - shorter scale lengths are easier to bend than longer
- String types:  steel string will be slightly harder than steel / nickel or bronze.  Pure nickel (more applicable to electric guitars than acoustics) will be a bit easier to bend than a combo of nickel / steel.  Nylon is easier to bend than all the others, but I suspect your question is more aimed at steel strings or steel / bronze for acoustics.
- One additional item others haven't noted yet is frets.  I've found higher frets make sting bending quite a bit easier than guitars with lower frets.
- Hand strength.  When I've gone up to a higher string gauge to get a bigger sound, it's taken me several months of playing almost daily to build up the hand strength (and calluses) to be able to play comfortably.

 
That

just bend em!@!
 
bend em like you mean it!
commit!
commit!
 
and don't bend sharp, like buddy guy does.
 
(i love that guy)

A few other things that affect string bending...

Action. Too low an action makes string-bending harder.

A Gibson-style stopbar tailpiece that's too low makes bending harder. Don't know why tailpiece height and therefore string break angle over the bridge alters string tension as far as bends are concerned, but it does. Too high a tailpiece is bad for other reasons, the trick is adjusting it until it works best for you.

Neck relief makes a difference as well, as does nut height on the lower frets.

Personally I don't find a tall fret height makes bending easier, but for me tall/jumbo frets have a negative effect on intonation, I suspect that might be because I fret with a higher pressure than some and when I press the string down behind a tall fret that bends it slightly sharp. My personally favourite guitars all have low, vintage frets and a moderately high action by shredder standards.

Electric guitars are very sensitive to how the musician plays, and what one person finds a dreadful setup is probably someone else's ideal. Within reason of course. An action where you can slide a pencil under the string at the 12th fret without contacting the string is generally agreed to be bad for anything other than lap-steel styles. Though I've come across two or three acoustic guitars exactly like that whose owners had been manfully (or womanfully) fighting the instrument for years. One insisted the setup must be "perfect" because that's how it was when they bought it new. Doh.

batsbrew
just bend em!@!
 
bend em like you mean it!
commit!
commit!
 
and don't bend sharp, like buddy guy does.
 
(i love that guy)

Buddy Guy is how I got hooked on Miles Davis!
 
I saw a PBS show in the summer of '65.
 
The first half hour was Mississippi John Hurt sitting alone on stage with a resonator, playing the delta blues like I knew from Gaslight Square in St. Louis. I'd hop the bus after my parents went to sleep and head down to G.S. and get a slice and a Coke for 25 cents and sit outside and listen to the old river blues until the last bus. I was in heaven.
 
The second half hour was this 17 year old kid named Buddy Guy, playing (very) high speed electric blues. Never heard anything like it. Love at first listen.
 
That Christmas, we went to Chicago and stayed with relatives in Elmhurst. Same situation. Adults asleep. I snuck out and took the train into the city to find me some Buddy Guy Chicago blues. Only problem was, it was the 23rd December and cold as s**t and no open windows or doors to hear music. Bummer. I saw one place on the east side of Wells in Old Town that had a line of peeps, so I figured...primo blues!!! Got in line, got in (at 14; not bad ), got a beer (to keep the waitress off my back) and found a place in a back corner.
 
And then the music started. WTF!!! That weren't no blues. WTF!!!
 
Well, long story short, it was Miles on trumpet, Tony Williams on drums, Herbie Hancock on keys, Ron Carter on bass and Wayne Shorter on sax. It was the second night (22nd-23rd December 1965) of their date at the Plugged Nickel. And 40 years later, I got all of it in a CD box set.
 
My mother never understood why I wanted to take up the trumpet. I was supposed to be a classical pianist. Ha ha ha ha ha.
 
But she got even. Once I was off in the Army, she gave away my MIA '63 Fender J bass and my old brownie Fender amp. 
 
EDIT: And my Farfisa mini. 

Apparently Steve Howe uses the same gauge string for both the top E and second B. Both 12, which is considered heavy for an E but light for a B. Obviously he never bends the top string, only the second.

From looking into flat wounds for my bass, it seems that the same gauge and same material can still produce different stiffness from different companies. The winding process is a factor too. LaBalla flats are stiffer than GHS

Another point I forgot earlier but would not really come into play on an acoustic is the width of the nut/saddle. The electric I use most has the narrowest standard nut, so a full bend can catch 3 other strings. Conversely, the widest nut I have only catches 2, so with identical strings the force for bending is significantly less on the wider nut.

On a typical acoustic, higher tension requires less distance to bend, but will still catch 2 other strings most often doing so. If only bending one string would be a lot easier.

When I played guitar in a band, I used ultra-light strings. Ernie Ball Super Slinky. Not for bending, though. It was because I'm a delicate keyboard player and will never need to pound nails with the callouses on my fingers.
 
The downside: I could never put fresh strings on right before a gig because they'd need 3 days to stretch enough to stay in tune. Plus I played a Rickenbacker 12-string, which was never quite in tune anyway. 

I've got nylon tapewounds on my acoustic bass. I ordered in new strings in from Germany for all my guitars, as there is little choice here. They were a gamble. They felt so floppy, like rubber bands when I first put them on, I thought I had made a mistake. They settled in quickly, and now I love them. When I go back to my Jazz bass they feel like steal bars in comparison.