I just finished beating ME2 on Insanity difficulty as an Engineer. It was pretty tough and I died a lot but I don't feel like it was over the top - pretty much just right all in all. I end up comparing the challenge of games I play to WOW raiding most times and it is a rare thing to find something as hard as some of the most brutal hardmodes in WOW. Certainly if I had a team of nine others playing ME2 and we all had to beat a particular encounter simultaneously without dying to advance it might be more comparable. I didn't have a team to worry about though and once I beat a particular fight it stayed beat; there was no necessity to retry again after playing perfectly to get somebody else up to speed. I don't feel like anything in ME2 was as challenging as really tough WOW hardmodes like Alone in the Darkness (best fight of all time imo) because mostly once I had the proper build and strategy they only took a small amount of practice, if any at all, to defeat. More than that though there was room for mistakes. I failed to cast spells for my squad right on the cooldown a lot and ended up letting my buffs and summons expire; no such foolishness is possible on really hard encounters.
One thing I did find strange was how the style of the fights drastically changed. The main things that change in the game when you go up to Insanity difficulty are the damage the enemies do and the additional defenses that are stacked on them. For fights where I was mostly just standing behind cover and shooting at other ranged units who were also behind cover the fights just took a long time. They rarely did anything interesting and as long as I played reasonably they would eventually fall over to be replaced by new spawns until they were all dead. The melee enemies were entirely different though because they were exceedingly deadly once they got to close range and doubling their effective HP made killing them before they closed impossible in many circumstances. It wasn't just a matter of taking longer; often against fairly weak melee creatures I had to use crazy kiting strategies, pull out heavy weapons, or do hilarious mind control tricks to win. On Grunt's loyalty mission I did all three at once! It is a mechanic any veteran WOW raider will be familiar with; if your dps is just a tad too low the adds the boss makes end up overwhelming you and even a small increase can change the fight from unwinnable to no problem.
I may be a bit of a masochist but I really long for combat logging and UI modification. I want to be able to custom build my bars so I can have big lights and buttons for my squad's powers and get enemy and friendly health bars in a more visually accessible location. I would be happy to test out weapons accuracy levels at various ranges against combat dummies and figure out optimal loadouts for any given situation; it drives me a little bit bonkers to see things like "+50% damage to armor" when I don't know what the base value is! 50% of WHAT? I would happily spend months tinkering away at a ME2 spreadsheet to sort out exactly how all the mechanics work but without any information to work with there is nothing to build. Not that it would help very much in any case because you use the Sniper Rifle at long range, Shotgun at close range, and powers on targets who are vulnerable to them so there aren't exactly a lot of decisions a spreadsheet could help with.
Not that the spreadsheet being irrelevant would stop me, of course. I don't examine the inner workings of games for advancement so much as I do it for love. Now I just have to figure out if I should start on ME3 and hope to get it done before Diablo 3 eats my life on the 15th.