So I've been dicking around in this a little bit, and so far I'm really rather impressed. Hell, this is something I would've been impressed by if it were just some little indie doodad in the bowels of Steam, but for a Flash game it's really something.
That said, there are still some... Things.
Couple little typos and instances of weird grammar here and there, plus what everyone else has mentioned regarding runes (getting different runes with the same name, runes that do effectively the same thing, getting a defensive rune with "+4 FP on defensive runes", so on), missing descriptions on runes and effects, stuff like that.
And it didn't make me feel much better losing my wreckingball dude to "You were killed by: I don't really know :("... Admittedly, I was probably abusing the dark arts a bit too much, but still... Messages make things better.
So far, it would seem that the Alchemist is by far the most powerful of the starting classes. Just toss that block rune of his into the Charge Cloud preset, and suddenly you're not only ministunning enemy units, but now you can spam the damned thing for 1 FP. With a few other runes, it just builds right up and out of control... Oh, yeah, I'm not sure if it's intentional, but Greed FP usage gets calculated separately from things like Split and Blood runes, and also Fluxfeed I think. So if you have a Blood rune attached, you'll do the original FP in damage to yourself, and then pay the +100% of Greed in normal FP. I wound up using this to good effect in my last run, as it allowed me to keep the effective FP cost at 1 per cast while also using a Blood rune and Cure rune to spam the enemy into oblivion and myself to greater and greater heights of regen.
It's probably just me, but the Draconian seems like he could really use some extra FP or starting runes that lower his expenditure or something... Unlike the Pugilist with his passive buff, the Draconian can really only kill things by using his abilities, and those abilities are short-ranged and VERY expensive for what they do and how often they need to be done.
I got the click-click rune by beating whatshisface, and noticed that it was completely useless to me since I'd gone... What was it, 14? ...floors by that point, and had yet to encounter anything that hinted at a shield effect. Either I'm missing something, or the drop rate for that effect is dreadfully low.
And for those who don't yet have the Absorb rune... Feel free to abuse Bump. Create an extremely high-health block, eat it, suddenly +45 power to all spells! Cast self-empowering buff you designed earlier... Well, wouldja lookit that! 169 extra power levels!
Aside from that, only thing I can think to comment on is the music, which sounds like some supermarket shuffle at times. ...which, honestly, is kind of appropriate considering how you're always sniffing about after more, better runes.
EDIT: Just beat it, all 25 levels and special boss battle of it. I'm sure the final boss would have been a bit more imposing were it not for the fact that I pinned him down with an exponentially damaging cloud that, after a couple castings, was doing more than five times his total health every tick... Perhaps some teleportation is in order, so he's at least not sitting still the whole time?
EDIT2: Doing a pugilist run now, doing okay. Just had my FIRST EVER shield rune drop! Aaaand... Well, not entirely sure if it's intentional or not, but an active shield also shaves damage off of the Sacrifice effect. Reaaaally helps as far as keeping flux levels down goes.