If 4e were going to be just a simple streamlining of things like grapple, I would be totally happy with the amount of external playtest WotC is planning on. It's not going to take too much to work out a single element of the game and find its issues, especially if the way its resolved is being simplified.
But from what we've been told, 4e boasts the following:
The game goes up to 30
The way monsters are statted and created is changing
The magic system is getting a huge overhaul
We're getting some kind of maneuver/magic system for non-casters
Races are being overhauled
That's too much. Too many very big changes that could interact in thousands of little ways that the designers might not have foreseen. I don't know that they're enough to justify the same length of time as went into 3e's playtest, but it's certainly enough to demand more than a couple of months with a diverse set of eyes looking at it.
I don't feel like WotC has burned me, and I don't think they're a bunch of wild incompetents who just can't see the transcendental brilliance of my own house rules. ...however transcendentally brilliant my house rules may or may not be.

They are professionals and they've earned some credit from their track record. They've convinced me through actual play that some of the things they put out which appear unbalanced really do work pretty well. But this just isn't enough time. Even gifted professionals don't foresee every exploit or every gap in the rules.
I don't know whether sales projections are forcing a quick timeline, Hasbro is leaning on WotC, or the designers are just so sure of their own work that they don't see any reason to plan for incremental revisions in the playtest process, but whatever it is the short timeline is very worrisome. It suggests that large portions of the system simply will not be changed, even if major faults are discovered. They don't have the time to make or test those revisions before the books are off to the printer
The volume and length of playtesting for 3e did much to build confidence in it, especially in the face of major paradigm-changing innovations. The lack of the same here works to undermine my confidence in the very same. I understand that as soon as WotC announced, it was signing itself up for a big decline in sales of 3.5 material and that would push for a late announcement when the game was virtually finished. That can't be helped, and I don't want WotC to cut its own throat. But if they came out and told us that 4e had been quietly in secret external playtest for seven or eight months already and would continue to be so for some months afterwards, I would feel a lot better about things even aside my major disagreements with the direction design philosophy seems to be going in.