The obvious things
Next most obvious is programming is all done by text. This means you don't have anything resembling Construct 2's visual editors. Instead of visually designing levels, often programmers resort to typing in long lists of object co-ordinates. Then, imagine setting an object's collision polygon by typing in each point's co-ordinates, or writing code to run animations! Some programmers get around this by writing their own tools to help, but then that's even more work on top of designing your game! You could do an awful lot of work on your tools before you've even started the game. Construct 2 has these visual tools built in already. And imagine if you didn't have any tools - it's incredibly hard to be productive typing in lists of co-ordinates, and hardly intuitive.
Don't forget Construct 2 helps you encode all your audio to both .ogg (all OSs) and .m4a (Windows 7+ only) to help reduce the download size and cover all browsers, since there isn't one format that works in all browsers.
The less obvious things
There are even some low-level technical advantages too. Suppose you've braced yourself: you're going to learn the language and its quirks, design your own tools, write all the math and movements, encode audio yourself, and so on. There are still two more things you need to get right.
One more thing - do you want physics in your game? Have you got code to decompose concave polygons in to multiple convex polygons so physics collisions work correctly? Have you learned about Box2D's fixtures, bodies, joints, correct world stepping, and so on? If you want physics in your HTML5 game, that's another whole can of worms.
Construct 2 does it for you