I'm not going to get into debates about how to balance the system, and I haven't tried the beta, but I'd like to say a few things about the idea in general. (this became a rather long post, just skip to the last paragraph to see my main points if you don't give a crap about my experiences with Elements)
Firstly, I started playing elements in mid-march, so a bit over 3 months. (say roughly 100 days) My stats are 77764 score, 2393-1941 win/loss, (I was unemployed in march and april :p) 3080 electrum, and I own 414 rares and the complete set of unupgraded cards, except that I need one more druid staff. I have no idea if this is good or not, but I'm using it as a point of reference so people see where I'm coming from.
That score, if I remember from the list, is level 58, and the upper end of gold. I don't know what level zanzarino thinks an average person will get to after 3 months, but that's what I have. I suspect that due to having more time in march/april than some people, and having played CCGs almost as long as CCGs have existed (I started about 1995) that I learned Elements faster than most.
Now, when I started playing Elements, I thought it was great fun. I eagerly took on the simple n00b quests and I can remember trying to farm that first 500 score for the rare weapon :p Then I started taking on AI2, and eventually AI3. I tried moving on to higher levels, but then at that point I got a bit frustrated with losing to AI4/5/6 so much, so I sold everything but a 30 card monolife deck, (rares were worth like 150-250, seemed HUGE at the time!) upgraded as many cards as I could, and farmed AI3. At that point the game got a bit boring, but I slugged away at it, playing hundreds of 2-3 minute games and reinvesting in the life deck until I was wtfpwning AI3. Then I built scaredgirl's rainbow deck and started playing against AI5/6.
At that point, even AI5 was still a bit of a challenge, but it was exciting to play against multiple draws and triple marks, not to mention all of the upgraded cards. And you could actually WIN upgraded cards if you got lucky! I still remember winning my first upgraded card (it was a relic, oddly enough, given how rare it is to win a relic off a false god) and going HOLY F... I can sell this mofo for 1k+. Great fun! Eventually I progressed to building my own rainbow deck to farm false gods, and slowly improve my win rate. I invested in an upgraded cremation deck to farm t50 and I now have all the unupgraded rares except one druid staff, and in most cases 12 of each weapon. (not yet with 6 upgrades in all of them, though some I have that) t50 got pretty boring for me and I just don't have the motivation to get the rares left that I need; I'm content to pick them up off of false gods when they come, though at some point I'll have to get that last SoD.
Farming false gods was great when I started that, too. The decks seemed SO hard at first. (some still do :p damn you eternal phoenix) Some of them were pretty easy even then (hi neptune) but most were quite challenging. As I farmed more though, more and more of the false gods were easier, and the day finally came when I even beat Divine Glory. (the last one I hadn't beat) I still play against the FGs, because it's the hardest level, and have a lot of fun, and of course I'm still working towards owning a complete set of upgraded cards, Eventually, though, I'll have that, too. (probably not for a few more months yet... how many rares would a full set be, anyway? I'd guess 1200 or so, and then however many towers/pendulums constitutes a 'full set', so another few hundred cards)
In any event, farming false gods will at some point become less interesting. Why? Because I'll be winning too much. It won't be challenging. At this point, my win rate against FGs is about 50%. The best claimed win rate I've seen so far is for GotP Time at about 66% I think. Presumably, that means I should be able to achieve at least a 66% win rate, either by copying that deck or finding something equivalent or better, if it exists.
From what I've read about the platinum level of the Arena, it will be significantly harder than false gods. I'm assuming that most people in the platinum league will know how to build a good deck. If they just grinded score forever and don't know how to build a good deck, they probably won't submit a crappy deck because it would cost them electrum, so I'm assuming that all of the platinum decks will kick some serious ass. And now I finally get to the core of what I'm trying to say, which is that the Arena will add an even greater challenge, and something to work to beat. Will we lose a lot against platinum? Yeah, probably. Will we get better rewards than false gods? Maybe; I have no idea and actually don't even care that much if we do or not; I'll leave that for others to balance. The biggest thing that interests me about the platinum level is that it will probably kick the hell out of me. I like that. I like a challenge, and to a certain degree, we all do. I mean, does anybody take a fully upgraded deck against AI0? No, other than maybe once as a joke; nobody would seriously do that, because it would be no fun. Now, false gods will never be that easy for anybody, but at some point they will probably be easy enough to be uninteresting, once I can build any deck I want. Platinum level adds that new dimension, something EVEN HARDER than false gods to fight against. Even if there weren't any rewards for winning at all in the arena, it would still be fun as the ultimate test of deckbuilding/playing skills.
(well, the ultimate challenge unless zanzarino designs stronger false gods or adds another level of upgrading for 15k a card and makes 'true gods' with full doubly-upgraded decks or something :p)