Fourth Edition D&D is a game that has its point of strength in tactical grid combat.
While in previous editions it was stated that you could do anything you wanted and it would have worked, most of this was true because the dungeon masters added their explicit or implicit rules and rulings to the existing, written system specifically to be able to manage those different things.
In D&D 3.5 there are no rules for political scheming, for romance, for investigation, just as they are not present in D&D 4e.
4e just stopped saying you could do all those things with ease and focused on a game that could be played by a party of heroes.
Its rules help you have significant combats, favoring a step-on-up style of play.
(Step-on-up is design terminology for a game where the participants value using skill and fortune to overcome obstacles more than writing a satisfying story or experiencing a certain atmosphere.)
Is 4e mechanically driven?
Yes, it is. As is D&D 3.5, as is Pathfinder.
While you fight enemies, the vast majority of the things you do can be summarised in "My character moves there, strikes for 24 and hits for 12, next". There's no incentive to narrating things and (unless the DM wants it so) there's no real impact on the game if you charge with an evil grin or you charge with a determined stance or any other descriptive fluff.
Since it's not important and it takes up time, people don't usually narrate conflicts that way, treating combat like a miniature game, save for maybe certain choices that stem from their character's personality.
For clarity, quite at the opposite side of the spectrum there's Dogs in the Vineyard, where you use a poker-raise-like system but you need to tell to the opposition what your character is doing because if you don't describe your actions and feelings it's not possible to give back a proper response.
(DitV is a game that doesn't focus on combat but on moral choices.)
Will the players be constrained by the dice?
Not so much. They could fail a diplomacy check, just like they did in previous editions. And, well, they can't exactly narrate that they were successful despite the dice, but that's the point of rolling a die, isn't it?
But there's nothing forcing you to change your character - which could also be a downside since most D&D characters have, in my experience and in the experience of several people I know, the tendency to remain rooted to their background morality, which can be unattractive for some players.
Sometimes I see players complaining of a very similar thing. It was common use in previous editions and in some RP groups to handwave many things. A combination of create water and frost ray could have the additional effect of creating a slippery surface, electrical effects could have a better result if cast in water and so on.
D&D 4e has its powers (including spells and martial maneuvers) strictly defined in a mechanical way to prevent some sort of combo abuse we've seen in 3.x edition.
This also means the powers are supposed to be used for dealing damage, for inflicting statuses or for whatever they were designed for, with little space for imaginative uses.
I personally believe this is a good way to wrestle the game control out of the hands of the player with the better imagination or the better oratory and give everyone the same possibilities to shine, but some players (usually the ones with better imagination and oratory) find this unfair.
I finally had one of these players telling me he just doesn't want to let that power go.
Character planning
Keep the concepts, start from scratch.
Character creation in D&D 4e is way easier, especially for spellcasters. There's fewer bad choices and they are more obvious than they were before.
Many things can change. A fighter that wants to charge is better built as a barbarian and refluffed as a fighter (4e encourages refluffing).
Multiclass options work very differently in 4e to prevent dipping, and prestige classes now can be gotten at lvl 11 no matter what.
The main criticism I'd be prepared to receive is how the chassis is the same for every class. BAB goes up every two levels for everyone (but touch AC is no more a thing and armors scale differently), there's no more iterative attacks save if a power calls for it, even warriors need to choose several powers that can be used at will, once per encounter or once a day.
Pre-buffing is no more a thing, clerics don't wear heavy armors... yeah, some things have changed. Better to start anew.
Mechanical fitting
There's a whole new bunch of monster manuals (the first two are recognized to be "old design", with monsters that deal less damage than they would do, others with more HP than they should have and bosses easily permastunnable).
Instead of having monsters behave like characters (with the known problem of having a set of useless abilities that PCs use and monsters don't) now they are built to do their job in combat and maybe in social situations. They have few powers that synergize with each other and that are all useable in a single combat. Instead of leveling up, there are several different leveled kobolds, orcs, gnolls and so on.
Refluffing is your best friend.
Gods are less important in D&D 4e, except for clerics, who get an (usually unimpressive) extra choice for their channel divinity class feature if they get a feat tied to their chosen god.
Paladins are the champions of their god, whatever their alignment is.
So if you had some gods you want to use, just decide which channel divinity feat to assign them and you should be OK.
Maps; maps have bigger rooms. Encounters usually sport lots of creatures, most of which will be minions: 1hp creatures that can damage and flank and are dangerous in numbers, but easy to get rid of.
Just like 3.0 maps were unsuitable for 3.5 because of the changes in the number of squares occupied by large creatures, you'll need to get bigger rooms.
Rooms with pits, pools of acid or lava and the like are better because a lot of the tactics can revolve around forced movement (not 3e's "I don't attack to push you" but 4e's "I push/pull/slide you while attacking").
On the length of combats
Yes, they are lengthy. I've played 2-hour-long combats at level 1 and I've been told it gets worse at the higher levels.
A common houserule to stop that is to double all the damage for everyone.
I've also had 7 hours long level combats in D&D 3.0 and I don't think Pathfinder has solved that problem yet, so I think it depends on the enemy team setup selection.
