You don't really need to adjust solos from the MM1 if you don't want to; they'll pretty much work fine, though most have some limits that way.
If you want to adjust them to bring them in line with DMG2/MM2 guidelines, basically drop defenses by 2 and, if the monster is paragon or epic, reduce hp by 20%. You certainly should NOT drop their damage imo; most MM1 solo monsters do low enough damage as it is.
If you want to tweak them to make them more fun, that's a real case by case thing, but I agree that minor 1/round actions and immediate action abilities both really help a solo present a credible threat. I also really like my "fast & furious" variant- half hit points, double damage dice (so 2d6+3 becomes 4d6+3). That makes solo (or whatever) monsters more dangerous but also quicker to fall.
Disclaimer: the party I have run 4e for has multiple leaders in it, so my perception may be colored by their extraordinary healing ability.