Let me start out by saying I agree with Dobbler that rotation might become necessary in the next year or two (if that is indeed what he is advocating). I disagree somewhat with his logic, but ultimately come to the same conclusion. Here's my thinking...
--Issue: Is price a barrier to entry that affects competitive players? I really don't know how true this is. I don't mean to say that price isn't a factor, but the extra $300 a competitive player might have to spend given the larger card pool is a drop in the bucket compared to the travel expenses associated with attending tourneys. Also, if these costs can be mitigated by sharing cards (and they can), a lot of competitive players will share with meta-mates to reduce the costs, even while they fork out wads of cash for airfare and meals away from home. I'd be surprised if this is really an issue at the moment. The costs of the LCG are already significantly less than they were in the CCG (considering rare chasing, etc.), but I don't remember many people complaining a lot back then. Three years from now it could be an issue though...there's definitely a line somewhere.
--Issue: Rotation affects competitive players only? Agree with Dobbler that rotation first and foremost affects tourney-goers, but disagree wholeheartedly that it ONLY affects them. Rotation, or any ruling/FAQ, transcends the competitive environment to affect many casual players. In large part, this is because the "official" ruling is often assumed to be the "right way" of doing things. So if there's an errata to a seemingly overpowered card, casual players can ignore it, but I strongly suspect that most casual players adhere. Here's a test: How many casual players have you met? Now how many of them regularly play Jaquan, which is probably *the best* casual card in terms of being fun, powerful, and banned? Extending this logic further, and I think it is appropriate to do so, suggests that many casual players will similarly adhere to other legality rules, even if they never play in tournaments. Part of this could be that their meta-mates or friends play competitively and don't want to build "casual decks" with different rules. But mostly, I think it has to do with players assuming that there's a reason for errata/ban/restriction or even set rotation. Anecdotally, I find that my metamates who never attend tourneys (which is about 1/2 of them) still all follow legality issues.
--Issue: Rotation would make a lot of currently useable cards unusable (ie the "seasons deck problem")? I think this may be overstated. Obviously seasons builds would change significantly, and not all houses would be equally equipped to deal with them. That's true with any house and/or mechanic...shadows, knights, etc. On the other hand, this would free up new slots for decks as well. Some old cards that are not particularly useful now would be much more useful. For example, rotating Venomous Blade would make all kinds of 2-STR characters much more attractive. Or to use a card not currently on the restricted list, rotating Game of Cyvasse would free up another slot for one of the many great Martell events that never sees play (many fail to make the cut even with very powerful effects).
--Issue: Would rotation add more variety or less? This is where I think the primary debate should be. After all, if we can increase diversity without printing cards (by doing the opposite, in fact) then we should explore this option. Before people post gut reactions about how "removing cards removes options," let me explain. There are certain cards that when played in combination with each other become environment defining. Wintertime Marauders + winter in GJ is one of those...other archetypes include Targ burn, Lanni kneel and Martell control effects (including Venomous Blades). In fact, many people complain that design has favored "control" decks over "aggro" since the transition to LCG. Rotation presents one possible option for a sort of environment cardpool "soft reset" that might plausibly add balance and variety to the environment. (I don't mean to say the environment needs rebalancing...that's a different discussion. All I'm saying is that it is possible under certain situations to expand diversity by restricting the cardpool.) The bottom-line question then, I believe, is would the competitive environment be more diverse (and thus likely more fun) with fewer cards in the card pool?
I don't know the answer to this last question, but my gut feeling as that at some point in the next two years we'll reach a point where power creep (even if only because more cards that compliment each other, ie "synergize," are printed) begins to reduce deck building variety at competitive levels. At that point, some restriction on the card pool (beyond specific card "restrictions") will add diversity. I don't think we've reached that point quite yet, but when we do it would be a bad idea to rotate immediately. I believe the best approach could be to give players 1 or 2 years notice in advance that X cards will be rotating out. It's much easier to stomach a rotation announcement if it's 1.5 years down the road. I could be wrong about the need for rotation...but either way I think the main criterion should be about creative deckbuilding and environmental diversity, not dollars.