Re: Arena mode. They've already announced that they're working on an asynchronous sealed mode which should fill that void very nicely whenever it's finally ready.
As for the announcement of Set 2, I have mixed feelings. By the time it launches, collections will have been live for 4 months, which is a totally reasonable pace of development for a TCG that depends on shaking things up on a regular basis to prevent the meta from getting stale. Letting their core base of players get bored certainly isn't a good option. But of course, it's still in closed beta, so not everyone will have had the chance to play in the Set 1-only environment. Really, the problem isn't that it's too soon to launch another set, but that the features necessary to move into open beta are so delayed.
In retrospect, it's not really surprising that a company with a core background in physical games and experience running the WoWTCG would be able to keep up a steady pace on the new card pipeline despite running into delays on the software features. In an ideal world, PvE, trading, double-backs, open beta, etc. should all have been launched before Set 2. But in this situation where they just aren't ready, I think it's the right call to keep the core PvP game (their minimum viable product) humming along rather than letting it stagnate while waiting for everything to come together.
I'm impatient for PvE too and anxious about the lack of visible progress, but I'm willing to believe them when they say the delay is more of an issue of ambitious scope leading to design and implementation challenges rather than a lack of prioritization. You might read this if you haven't already: http://coryhudsonjones.tumblr.com/post/83645967847/hex-beta-is-almost-here
I get this psychologically, but it doesn't really make practical sense. Limited formats are a level playing field regardless of the size of your collection. Casual constructed decks are pulled from whatever you happen to have rather than 100% optimized, and a collection with mostly set 2 won't be any weaker than one with set 1, especially since you can easily pick up common and uncommon playsets of set 1 to give you a much bigger pool to draw from. And if you get serious about competitive constructed, you'll likely need to pick up singles for the decks you want to build from the auction house; in that case you're filling 60 slots and it doesn't make a huge difference whether those slots are pulled from a total pool of 300 or 1500 cards. The only time it's a real problem is if you're trying to collect full playsets almost entirely from buying and opening packs, which is a terribly inefficient approach to TCGs at the best of times.
Any game hoping to maintain and grow a community over a period of years has to ensure that things are approachable for new players, and that's the purpose of the block system. Having a few sets in rotation is definitely manageable -- if it wasn't, no TCG would have ever gotten off the ground.
Besides all that, there's every reason to expect that the game will be more fun and varied in both limited and constructed when the meta is drawn from two sets rather than one, so if you happen to start playing a bit later, you'll have a better first experience than the early adopters.
Also, not sure what your "more finished" benchmark is, but if you are interested in drafts, there's no reason to keep holding off right now. Games, packs, drafting, deckbuilding, and collections are all functional, stable, and enjoyable.