A very basic start, but it's a good framework for later variety.
Somebody already mentioned the need for a mute button. The graphics are certainly very basic, but those can wait to be changed until the gameplay itself is adjusted. When you DO decide to switch out the art, I would recommend making it something other than a frog theme. Frogger is fun and classic, but by changing the "setting", it seems less derivative even if the core gameplay concept is the same.
It would also be nice to get multiple lives. 3 is pretty common for arcade-style games like this.
In this current version, every time the frog reaches the loot, the red blocks move a bit faster. For the most part, this just means you follow a pattern of ducking in behind the next row and following it to the other end, stepping forward, and tracing along in the other direction. It gets trickier as the speed increases, but it's mostly the same process.
I have a couple suggestions that might feel like too much of a change from the existing game, but just see what you think. I'll refer to each loot increase as a "level" even though it's one continuous flowing game (I think it was a great idea to go up AND down instead of just teleporting the player back to the bottom every time!).
In the first level, I would have slower (and fewer) red blocks. Maybe some rows are completely empty at first, just to encourage you to test out your movement speed.
Principle 1:
Imagine this: you first start the game, the first row and the last row are empty, and there's only 1 red block on the other lines, and they're sitting still. After you get 1 loot, the blocks slowly start moving. Since the "endpoint" rows are empty, a hurrying player has a split second to realize something changed so they can stop or back up to avoid bumping the now-moving blocks.
Next time you get a loot, a block is added to the row on the OPPOSITE side (same reason as before, you don't accidentally bump into something you didn't know was going to be there), so it's not empty anymore. Gradually increase speed or add more blocks (or both) each time you get a loot until it's more like what you have now. By making these changes more incremental, you can increase the number of "levels" the average player can reach, making them feel like they're making more progress and "getting more" out of the game. That's not essential, but some players may prefer it. It also allows for more milestones if you end up getting Medals added in sometime later.
Principle 2:
Different kinds of guardian blocks. Instead of all of them going up a speed when you gain a level, maybe only some of them do. Maybe some get faster and some get slower, or even stop. Imagine trying to navigate between some fast-moving blocks and then you suddenly have to tell your brain to put on the brakes so you don't run straight into a slow/stopped block.
Maybe at a certain level, a row of blocks changes so instead of looping in the same direction, it starts going back and forth. This would help break up that player movement pattern I mentioned at the beginning. If you did something like that, I would recommend making those blocks a different shape, or at least a different color (maybe something lighter, so colorblind people can still tell them apart)
Principle 3: Stage progression
This ties closely into that "slow start, gradual difficulty curve, bigger high scores" I mentioned earlier. Maybe the first 5 back and forth "levels" are designed to get you used to the movement and the idea that obstacles will be getting harder, and then you get some kind of "stage clear" screen and move to the next (albeit very similar) board. I just use 5 as an example, it could easily depend on how difficulty would scale. I would use an odd number so that the character begins at the bottom and ends a set at the top. Each set of 5 levels would have a different challenge to learn and apply.
Examples:
set 1. absolute basics. learn controls, learn that guardian blocks should be avoided, learn that guardian blocks can move (all in same direction at first), etc
set 2. rows don't all go the same direction anymore.
3. rows don't all increase speed at the same rate anymore
4. some rows get faster but some get slower
5. some rows go back and forth instead of looping
6. sometimes there's a guardian block (very visible!) in the loot row, so it's not safe to sit there a really long time.
7. some small portions of some of the grey rows are blocked, so you can't jump to the next row at that particular spot.
8. Some rows cycle through speeds every time they loop instead of when you get loot (slow, med, fast, slow, etc) - maybe have color/lightness cues that change with these speeds.
9. Some rows have just one guardian block that sits still until you get into its row, and then it moves toward you.
10. The room is dark, and you have a glow radius that lets you see a circle around you
11. who knows?
Breaking the game up into segments like that would allow you to change up the art as the player progresses (you could do that anyway, but it would be less jarring if there was some breathing room in between). Like, if it was a medieval fantasy theme, the player sprite could be a knight or something, and the different types of guardian blocks could be different kinds of monsters or enemies. The room graphics could change in each "world" too. Like maybe you start out on a farm, and then you get into the small town, and the home castle, then the forest, mountains, canyons, ice caves, volcano, etc. to some made up villain's castle. The same concept applies if you wanted the setting to be scifi, or animals, or ghosts, or completely abstract.
Concept 4: collecting
Maybe the real "loot" is a coin (or something) that is placed somewhere on one of the rows. You have to collect the coin, or else reaching the end of the board won't trigger the next step, you have to go back and get it for it to count. Then for whatever is the final step in that section (number 5 if we're following my example), instead of a coin it's a key. This gives the player a visual signal that they're about to advance to the next area.
Like I said, these are just spitball suggestions, you can consider any or none of them as you feel. My main recommendation is to get more variety in there. I'd like to see how you can take the basic concept of Frogger and transform it into something we haven't exactly played before.
Good start, good luck!
Bonus bug info: If I use my scroll wheel to go down the page while the game is running, I come back up to find that the distance between various red blocks has changed. Not sure if that's a flash thing or what. It doesn't seem to happen when I use the scroll bar in the browser window.