Hello there!!
First of all, I would listen carefully what the gurus just said above, specially the technique on how to play with the clutch lever that Luca described. I found it very helpful whenever my 1st gear doesn´t want to kick in...
Anyways, just in case, about a year ago I had the exact opposite situation that you describe... all my gears were making an ugly sound and feeling low powered but the 2nd gear was perfect.
What we found (after opening the tranny) was that the plastic roller that indexes the gears in the selecting mechanism (thanks to Wirespokes for explaining that to me), had an oval shape which produced the awful sound by the incomplete gear setting and a fake neutral as well (my bike was runnning in neutral, imagine my face!!!).
Good news were that we were able to get a new roller locally (I live in Central America, so parts for old bikes are rare) and, all the gears were in perfect shape, no nasty metal mud.
Here is a picture I found on the net, showing the little bastard!!!
I hope you can resolve everything without opening the tranny, but if not, hope this helps!!!
Take care!!