Watched MI1 and MI2 last night (on sale on iTunes so I completed my MI collection).
MI1 is definitely the outlier in the series as far as style and tone, it's definitely a different feel and some different characterizations. It's also earlier in his career so Hunt wouldn't be the same character he is now, but he felt much more "fun" and the whole film had more of an old school spy feel. Technology is a limiting factor for obvious reasons, but there's still a few great scenes (the famous ceiling sequence in particular).
MI2 feels a lot more like the newer installments, it's also a John Woo flick so Hunt went a bit overly ninja in a few scenes

Definitely a lot more action and more of a modern feeling threat (bioweapons) but the Woo influence made it feel like a pretty big tonal shift from the first film.
Will probably try to revisit MI3 tonight, I've seen 4 and 5 pretty recently but might as well keep going I suppose.