I'm currently playing through N+ on my PSP, and I hate to say it, but there are several factors about the game that I absoloutely hate.
1) The new sounds (basically, everything that isn't teh gold pickup sound) are total stuff and I preferred the old ones, it's almost impossible to hear the chaingun firing.
2) The new graphics. Again, I much preferred the old, simplistic pixel graphics rather than these fancy shaded ones.
3) The way that some levels are harder than a metric assload of stuff, and some are just way too easy to be in the later levelsets in the 3x's.
4) the loving scrolling, that just does my head in, especially when they actually designed the stages to be one or two blocks bigger than the PSP's screen, just to use the scrolling.