You think Flash is bad now, you should have seen it 5 years ago. It currently has a range of 425 (or is it 400? The wiki is contradictory) and a cooldown of 300 seconds (5 minutes); it used to have a range of 450, a CD of 180 seconds (3 minutes) and it used to pop projectiles when used (unsurprisingly Veigar hated it)! If you go back even further (to the start of the Alpha) it had a range of 1000 and wouldn't interrupt channels, so you could start channeling Absolute Zero then blink right on top of the enemy from effectively offscreen in time for it to detonate (although it DID initially have a 0.5 second cast time)!
Unfortunately, the spell is embedded in Lol's design like a barbed hook. If a champion's kit doesn't have any kind of jumps or dashes then Flash is essential simply because there's no other way to cross walls. If a champ DOES have a jump or dash, then it's still essential to let them keep their edge over every other champion that has to take Flash, plus it lets them jump twice to get major surprise range.
The spell has somewhat balanced itself out in the long term- thanks to its long cooldown, getting your opponent to use their Flash without burning your own can actually be considered a win in itself. 2 games back, I ganked the enemy LeBlanc mid as Sion at level 3, waiting in the brush until she Distortioned then wading towards her and forcing her to Flash. I knew full well I had NO chance of killing her, but I got her Flash so I counted it as a successful gank (my Viktor thanked me for it).
Personally, I think it wouldn't be terrible if it was reverted partly to its original form by giving it a brief cast time (maybe the same 0.5 seconds it originally had). That way it'd be less of an "instant escape" while still giving you the functionality to cross terrain or close from out of range to within striking range near-instantly. Won't happen, though.