I'd love to see manual setting of diff as an option again.
Anything below 2.3Ghash pointed to slush and you'll keep getting sent diff 1 shares. Of the other pools I've looked at 2 is set as the bare minimum.
Your miner is not sent shares of any difficulty - in fact, it is not sent shares at all. It is allocated new work every so often, and told by the pool what difficulty of result is acceptable. When it calculates a result whose difficulty matches or exceeds that threshold, then the result is sent, and you are credited with a number of shares equal to the threshold difficulty.
Let's say you are mining with a GPU producing on average four results per minute. Some of those will probably be rated diff 1. Slush has set your threshold to 1 (as it's so slow) so each of those results is accepted and earns one share. If you were able to set your own threshold, as used to be the case, any results below the threshold you set would not be sent and thus ignored. You might then go a minute or more and earn nothing, even missing out completely on some very short rounds where you might otherwise have had at least one or two shares.
You now upgrade to a faster miner, producing around 150 results per minute. Slush will quickly adjust your difficulty threshold higher, aiming to receive around 20 results per minute. Let's say it's raised to 4 - then your miner will only send results with a diff of 4 or more, and you will be credited with 4 shares per result. Higher, but fewer of them.
In all cases the work allocated is the same: it's just the difficulty of the accepted results that varies.
The whole point of this being done automatically is that it optimises the rate at which your results are sent, preventing the server being stressed by excessive traffic, and using less of your internet connection bandwidth, while maintaining your share generation rate. This latter can be important, for at busy times your connection speed may vary widely, and you might find that your hash rate slows down noticeably; reducing the traffic sent may limit the damage.