Note that this has been edited to reflect the changes in Cataclysm / The Shattering, so the stat weighing is not final and a proper raid-balance of stats is not concrete.
Int provides you with spellpower and a larger mana pool, so it is naturally a strong stat.
Haste is also important, as it allows you to cast your spells faster, and to an extent, reduces the GCD (which makes a difference for instant cast spells). You get a good amount of haste passively through talents though (9% through Judgements of the Pure, 3% through Blazing Light, plus flat cast reductions on certain spells).
Crit is always nice to have, and it synnergizes nicely with some talents (Crit Holy Shocks reduce cast time on HL / DL, for example).
Mastery (at 80 and beyond) gives your heals a bonus shield equal to a % of the heal amount on your target. Shield effects have always been quite effective, and this more or less gives you a permanent Valan'yr effect to work with. I can see this being a very popular mastery, however we'll have to see how well it scales further on.
Spirit is now the go-to stat for mana regen as of The Shattering, and will continue to be so throughout Cataclysm. There may be a FEW items / enchants out there that provide MP5, but for the most part you're looking for spirit for your regen needs. It still remains to be seen just how much spirit is going to be needed to sustain throughout hardmode fights (especially with the ability to Judge Seal of Insight AND Divine Plea as decent mana regen abilities), but regardless, Spirit is the new regen stat.
As for leveling, just worry about Int, Crit, and to a lesser degree, Haste. Early on and leveling, a lot of healing will be done with Holy Shock and Word of Glory, which are both instant. Haste wont play a big role until later when you need to utilize your Flash of Light/Holy Light/Divine Light spells a bit more.
Short answer: Yes
Longer Answer: Yes, you can use them as long as you have the necessary skill for the item (i.e. you can't wear the plate chest with your druid). The EXP-Bonus will work (tested this myself with my cloth wearing druid ;) ).
However, the stats on said items may not be exactly what you are looking for, but I guess this is really obvious. :)
Best Answer
Average used bandwidth is very low. I haven't tested it recently but it used to be possible to run WoW off a 56kbps dial-up link, implying a download rate of probably 6kBps or lower. The latency tends to be a much greater issue, as creatures/players may not be where your client shows them as and it can impact upon spell casting delays, etc.
By the way, if that connection really is 256 mbps, that's faster than almost all of Australia and New Zealand plays WoW at - that is 32 MB/sec, and we tend to top out at 10-20 MB/sec.