Andrewsky, I'll do my best to answer your questions.
How powerful is .30-.30 compared to say, a 150 grain .308?
Within its useful range a .30-30 will be as effective. The .308 moves a little faster and that gives it a slightly longer point-blank zero, or easily achievable accurate range. To compare apples to apples, let's look at the Remington 150gr. Express loading in each caliber. The .30-30 has a muzzle velocity of 2,390 fps compared to the .308's 2,820 fps, or a 15% lower figure for the .30-30. The .30-30's muzzle energy is 1,902 fpe compared to the .308's 2,648 fpe, or a 28% lower figure for the .30-30. At 300 yards range with a 100-yard zero the .30-30 will drop almost twice as much as the .308: 28.8 inches versus 14.7 inches.
If you go by the generally accepted rule of thumb that one should smack a deer with 1,000 foot-pounds of energy to achieve a reasonably quick kill, the .30-30 will drop below that energy figure at a range of about 175 yards. The .308, on the other hand, will reach 400 yards before dropping below that level. That's academic, though: I've seen deer taken at 250 yards with both cartridges, and the deer didn't seem to know the difference! The .30-30 really is an amazingly efficient little cartridge, and its well-developed soft-point bullets do a very good job.
Also, don't forget that the average range for a hunter's shot in the continental US has for decades been below 100 yards. Sure, in Western states without brush or tree cover one must expect longer ranges and select one's rifle accordingly: but for typical terrain in the Eastern half of the country, the .30-30 has more than adequate performance. Here in Louisiana where I live it's very unusual to have a shot further than 50-75 yards.
(All figures taken from
Remington's ballistic calculator.)
Bear in mind that a more modern bullet such as that in
Hornady's 160gr. FTX LeverEvolution round will dramatically improve the .30-30's performance. The 160gr. Hornady load retains 1,000 foot-pounds of energy right out to 300 yards, and drops only 12.1 inches at that range - a vast improvement. With that ammo you're talking about virtually a .308 equivalent.
How many rounds would the magazine hold?
Most bolt-action .308's will hold four or five rounds in the magazine plus one in the chamber. Most 20" .30-30 rifles will hold five or six rounds in the magazine plus one in the chamber.
If you want to empty the magazine, do you have to do it one by one by cycling the action?
In a tubular-magazine .30-30, yes. However, some (e.g. the Savage 99) use non-tubular magazines. Those can be emptied as for any other magazine of their type, or removed if they allow for that.
Where is the safety?
Earlier models of lever-action carbines don't have an external safety. They have a half-cock notch for the external hammer, but that's not really to be relied upon. The safest method of carry is loaded magazine, chamber empty - it's very fast and easy to rack a round into the chamber as the gun comes to your shoulder.
Later models (usually produced in the late 1980's and 1990's) have cross-bolt or tang safeties.
Are the sights adjustable at all?
Yes, the rear buckhorn sight can be adjusted up or down and from side to side in its dovetail. In some lever-action rifles the front sight can also be moved from side to side in its dovetail.
There are many aftermarket sights offering finer adjustments; Williams and Lyman receiver sights, tang sights, ghost-ring sights (usually mounting to a scope mount or Weaver rail), etc. I particularly like XS Sight System's
rails for scopes (including Scout scopes mounted forward of the action) and
ghost-ring sights.
Hope this helps.