The vanguard's biotic charge was a fantastic ability that made combat even more exciting. The first time I used it my jaw actually dropped it was so cool. Zipping around the battlefield, sending opponents into the air in bullet-time and the up-close and personal shotgun action is an unbeatable combination. SPOILER ALERT: (After Jack gets free on her recruitment mission, the map is incredible fun as a vanguard. Multiple levels, cover and obstacles to charge through and duck behind). END SPOILER.
The main trouble with ME2 was the shared cooldown of abilities. It rendered many adept abilities (for example) a little worthless as there's simply no point or benefit to having two crowd control abilities, two damage abilities, etc as you can only ever use one at a given time. Also, because enemies with protection could resist biotic effects the added shields/armour at hardcore and above difficulties made most biotic abilities (including charge and the bonus ones unlocked from followers) much less useful and made the vanguard and adept (and presumably sentinel) substantially more difficult to play (out of line with the obvious difficulty level increase), adept even more so than the others.
Had a go on Infiltrator but gave up during Thane's recruitment mission. The cloak is alright, but nothing else works/recharges while it's on so it's not so great as a defensive tool, and playing whack-a-mole got old after a while. There's only so much uncloak->Sniper scope->headshot one can handle. Very much a one-trick pony.
Sentinel's superlative shielding and engineer's drones just don't sound like much fun and soldier, whilst undoubtedly powerful, can't do much more than shoot stuff.